Add the ability to specify a pack version in the pack resource

[mjtice]

Right now it looks like the pack resource is only validating the pack exists before the module installs the pack. Allowing pack version specifications would ensure a particular version always exists on the Stackstorm server.

Thinking out loud here...
Add version to the resource definition - something like:

st2::pack { 'backups':
    version => "v2.0.0"
  }

The version is passed along to this function which then executes a modified command

output = exec_st2('pack', 'list', '-a', 'ref', 'version', '-j', '-t', token)

Then if that specific version isn't installed it gets installed here, passing along the version.

This is a massive oversimplification, no doubt.