Support Optional Super User in Instance Create

The creation of initial users and/or databases is supported in an
instance-create request, but the creation of a super/root user is
not. Granted, it's possible to create or reset the root user, but it
must be done as a separate request, after the instance has completely
initialized.

Changeset: If CONF.root_on_create == True, a UUID is generated for
the root user's password and is returned in the instance-create
response as the "password" property/field.

Change-Id: I300d3f019766445b90411d5ab8651b1aa34a77e9
Implements: blueprint instance-initial-super-user
Closes-Bug: #1219624
Merge "Support Optional Super User in Instance Create"