added support to assign security groups

[bears4barrett]

Resolves this issue:
#36

[hartmantis]

Haha; that was fast!

The changes made the build sad, though :( Looks like a couple of style bits Rubocop doesn't like and the change to server_def causing several unit tests to fail. If those can be resolved and a test added for the new functionality, I'll merge this in.

[hartmantis]

Is it just me or does it seem a little ridiculous that this has to be handled this way, and that neither the OpenStack API nor Fog complains if you try to create a server with invalid security groups? Weird.

[bears4barrett]

I updated so security groups is not always in the server definition, only if provided. I also found that fog will catch the issue in the case of a wrongly named security group. The prior check has been removed. Lastly, I added a spec test for security groups.

[hartmantis]

Great! Thank you!