Make strutils.mask_password more secure

Make some enhancements to strutils.mask_password to allow it to
catch more cases of passwords in strings.

The following is a listing of patterns that will be handled. The
keyword that mask_password uses (a list of four now) is represented
by <key> and the password is shown as <password>. Quotes (both single
and double) are represented as <quote>.

--<key> <password>
--<key> <quote><password><quote>
<key> = <password>
<key> = <quote><password><quote>

All existing tests and patterns are still handled.

Cherry-pick from review.openstack.org/113407
Partial-Bug: #1345233

Change-Id: I693baaf225c5b393401d499d14f271cadb63eb2c
Merge "Make strutils.mask_password more secure"