Make keystoneauth reseller_prefix check for _

- Automatically add underscore if there wasn't one already.
- This makes consistent with tempauth.
- Fixes bug 1198878.
- This should probably need to be mentioned in the release changelog for
  upgrades.

Change-Id: Ia82f14f82c380c7ed2ddd190116f1493e410cfbe
Merge "Make keystoneauth reseller_prefix check for _"