User keystone policy to check if user password can be updated

Keystone have a policy for change password, we need to apply this to
horizon too.

policy:
  "identity:change_password": "rule:admin_or_owner"

Change-Id: Idd35646f9cbe4fadaa96bc85b243ed4f7823d9dd
Closes-bug: 1347354
Merge "User keystone policy to check if user password can be updated"