Enable pylint checks for "anomalous" string escapes

Enable pylint checks for "anomalous" string escapes

Escapes in python string literals are well defined, but can be
confusing.  These pylint checks look for backslash escapes in strings
that might be mistakes.  Two code refactors were required to satisfy
these tests:

1. midonet_lib.py used \**kwargs in docstrings.

There doesn't seem to be a sphinx standard for kwargs, so this change
simply replaces them with "kwargs".

2. Regex literals containing escapes replaced with r''.

The assumption with this change (and the underlying pylint
check) is that r'' literals are more straightforward for regular
expressions, where every backslash is important.

While looking at these regexes, this change also removes a few
unnecessary "\-" escapes.

Change-Id: I01528b2482f78b9e851685ebbf6fded4e58355f1