De230/disable server password fields

Fixes for Rally DE230, back ported from Icehouse.  Adds a new config option to hide setting root password on new instances, defaulting to hidden.  This should close out the bug without any config changes.

All tests passing.

Closes-bug: #1250713
(cherry picked from commit - db9c08e)

Backported-from: icehouse
Upstream-Review: https://review.openstack.org/#/c/56151/
Closes-rally-bug: DE230