Augment decking file schema to parse groups definition
Add custom validator for validating cluster definitions...

Previously we only supported lists of container names, but the decking docs say
say that decking supports either a list or a dict containing "group" and
"containers" keys. That makes things much more fiddly :-(
Make decking able to start clusters defined as a...

dict of "group" and "containers"
Split apart config loading and validation
Update tests...

Now test config parsing implicitly, by loading our base test config data from a
file at the beginning of tests.

Also, test we can spin up containers for clusters specified with the
"group/containers" syntax
Update dependency aware map to account for groups
Merge pull request #6 from ch3pjw/groups

Groups

Adds support for groups into pydecking. Reviewed by @leth in person, just writing it down here for completeness.