Add mask password impl from other projects

A couple of different projects have an implementation for 'masking'
passwords in logs and wsgi output.

This seems like something that should go into Oslo-incubator as a common
implementation. This implementation is based off the following two
implementations in Keystone and Nova:

  https://review.openstack.org/#/c/26487/12
  https://review.openstack.org/#/c/49664/

Partial-Bug: #1247217

Change-Id: I2b1bbb4d3ddaa163573b685b3e3a5e8a3977e390
Merge "Add mask password impl from other projects"