Adds integration tests for gce module.
Adds integration tests for gce_pd module.
Adds support for creating GCE persistent disks from images
Adds support for creating GCE persistent disks from snapshots
Adds support for attaching persistent disks to GCE instances
Changes cleanup_gce.py to use gce_credentials module.
Adds version_added property to new attributes in gce and gce_pd.
Merge pull request #6976 from clconway/gce-snapshots

Adds support for snapshots and extra persistent disks to the gce modules
Update version_added attribute on recent GCE module additions.
Make code more PEP8 compliant (if(x))

Remove useless parentheses, `if (x):` should be `if x:`.
Make code more PEP8 compliant (whitespace)

> Always surround these binary operators with a single space on either
> side: assignment (`=`), augmented assignment (`+=`, `-=`
> etc.), comparisons (`==`, `<`, `>`, ``!=``, `<>`, `<=`,
> `>=`, `in`, `not in`, `is`, `is not`), Booleans (`and`,
> `or`, `not`).
Make docker inventory plugin more Pythonic

There's no need to check if kwargs is empty before iterating over it.
Refactor CronTab.is_empty
Make code more PEP8 compliant (if len(seq))

> For sequences, (strings, lists, tuples), use the fact that empty
> sequences are false.
>
>     Yes: if not seq:
>          if seq:
>
>     No: if len(seq)
>         if not len(seq)