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)