Pin etcd to latest 3 version as st2 won't work properly with etcd 2

[arm4b]

Closes #58

See StackStorm/st2#4608 for more context.
st2 since v3.0 will require newer version of etcd which supports groups/memberships
See: https://docs.openstack.org/tooz/latest/reference/index.html#tooz.drivers.etcd3gw.Etcd3Driver

[arm4b]

Tried to switch to new driver, got:

$ kubectl logs sappy-seal-st2api-6566877d48-w2ft4
2019-04-08 19:59:43,772 DEBUG [-] Using Python: 2.7.12 (/opt/stackstorm/st2/bin/python)
2019-04-08 19:59:43,773 DEBUG [-] Using config files: /etc/st2/st2.conf,/etc/st2/st2.docker.conf,/etc/st2/st2.user.conf
2019-04-08 19:59:43,773 DEBUG [-] Using logging config: /etc/st2/logging.api.conf
2019-04-08 19:59:43,783 INFO [-] Connecting to database "st2" @ "sappy-seal-mongodb-ha:27017" as user "None".
2019-04-08 19:59:43,909 INFO [-] Successfully connected to database "st2" @ "sappy-seal-mongodb-ha:27017" as user "None".
2019-04-08 19:59:45,392 INFO [-] (PID=1) ST2 API is serving on http://0.0.0.0:9101.
2019-04-08 19:59:45,393 INFO [-] Creating st2api: StackStorm v3.0dev as OpenAPI app.
2019-04-08 19:59:46,307 ERROR [-] (PID=1) ST2 API quit due to exception.
Traceback (most recent call last):
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/st2api/cmd/api.py", line 82, in main
    return _run_server()
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/st2api/cmd/api.py", line 71, in _run_server
    wsgi.server(sock, app.setup_app(), custom_pool=worker_pool, log=LOG, log_output=False)
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/st2api/app.py", line 80, in setup_app
    router.add_spec(spec, transforms=transforms)
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/st2common/router.py", line 212, in add_spec
    __import__(module_name)
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/st2api/controllers/v1/keyvalue.py", line 437, in <module>
    key_value_pair_controller = KeyValuePairController()
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/st2api/controllers/v1/keyvalue.py", line 62, in __init__
    self._coordinator = coordination.get_coordinator()
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/st2common/services/coordination.py", line 227, in get_coordinator
    COORDINATOR = coordinator_setup(start_heart=start_heart)
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/st2common/services/coordination.py", line 192, in coordinator_setup
    coordinator = coordination.get_coordinator(url, member_id, lock_timeout=lock_timeout)
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/tooz/coordination.py", line 803, in get_coordinator
    invoke_args=(member_id, parsed_url, options)).driver
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/stevedore/driver.py", line 61, in __init__
    warn_on_missing_entrypoint=warn_on_missing_entrypoint
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/stevedore/named.py", line 81, in __init__
    verify_requirements)
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/stevedore/extension.py", line 203, in _load_plugins
    self._on_load_failure_callback(self, ep, err)
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/stevedore/extension.py", line 195, in _load_plugins
    verify_requirements,
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/stevedore/named.py", line 158, in _load_one_plugin
    verify_requirements,
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/stevedore/extension.py", line 223, in _load_one_plugin
    plugin = ep.resolve()
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2297, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/opt/stackstorm/st2/local/lib/python2.7/site-packages/tooz/drivers/etcd3gw.py", line 20, in <module>
    import etcd3gw
ImportError: No module named etcd3gw
2019-04-08 19:59:46,312 INFO [-] Connected to amqp://admin:**@sappy-seal-rabbitmq-ha-discovery:5672//

@Kami Looks like we'll need another pip dependency etcd3gw in st2 requirements.txt to make it work out of the box.

[Kami]

Yeah, as per my comment on Slack (etcd drivers in tooz are quite broken) - it needs a lot more work, not just driver installation so I put it on hold for a future release.

[arm4b]

OK works for me.
Leaving it as draft for now.