[WIP] Process host/group/inventory variables before request

[AlanCoding]

Resolves #564

This is more ambitious of a fix than I wanted this to be.

$ tower-cli host create --inventory=3 -n host1 --variables "ansible_python_interpreter=/usr/bin/python26" -v
*** DETAILS: Checking for an existing record. *********************************
GET http://localhost:8013/api/v2/hosts/
Params: [('inventory', 3), ('name', u'host1')]

*** DETAILS: Writing the record. **********************************************
POST http://localhost:8013/api/v2/hosts/
Data: {'variables': '{"ansible_python_interpreter": "/usr/bin/python26"}', 'inventory': 3, 'name': u'host1'}

Resource changed.
== ===== ========= ======= 
id name  inventory enabled 
== ===== ========= ======= 
15 host1         3    true
== ===== ========= =======