Do not create admin user if it exists already

In case if Keystone set to use some external authentication backend
(for example, LDAP), the admin user may already be pre-configured.
Need to avoid user creation in case if it exists already.

Change-Id: I06365619f5f21ef91b8de6fccefa13e56dc29d1c
Merge "Do not create admin user if it exists already"