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: I021976f1eafa881755c0abbe1f6ba1b546dee111
Closes-Bug: 1360232