Make user creation optional when creating service.

In some cases it is useful to be able to just configure
the service in Keystone and not the service user. This
is the case when e.g. a read only LDAP backend is used.
Added parameters configure_user and configure_user_role
(default to true).

Change-Id: If9bb802ff2bb0b3ece55f36df773059ba9c7e9de
Closes-Bug: 1360232