6 - Invert Host and HostGroup Dependencies

[curvedental]

• Remove 'members' definition from hostgroups template
• Add new 'groups' list to host definitions
• Update README to reflect changes

This addresses #6, as we have encountered the problem where we have dynamic host definitions and require hostgroups to be empty sometimes.

Note: This is not backwards compatible, as it adds the 'groups' list to the host definitions. If you would like, I can adjust my changes to be more backwards friendly but it may not be as pretty :smile:

[bcoe]

Hey @curvedental,

If you have the time perhaps you could take a stab at making things backwards compatible. We use this role for dozens of hosts at npm, and having to rewrite our Ansible scripts that consume this role would block me on being able to merge this pull quickly. I appreciate the contribution however! and would love to get it merged!

[astrumas]

Ill make some time near the end of this week and take a stab at it!