Add a method for changing a user's password in v3

The new Identity API v3 resource for changing a user's password in
keystone can be utilized with:

  c.users.update_password(old_password, new_password)

Change-Id: Ia887d853140a18ba75d6eb1bfde4ce2d64b7af60
Closes-Bug: 1239757
Co-Authored-By: Dolph Mathews <dolph.mathews@gmail.com>
DocImpact
Merge "Add a method for changing a user's password in v3"