ceph-dashboard: update create/get rgw user tasks

ceph-dashboard: update create/get rgw user tasks

Since [1] if a rgw user already exists then the radosgw-admin user create
command will return an error instead of modifying the current user.
We were already doing separated tasks for create and get operation but
only for multisite configuration but it's not enough.
Instead we should do the get task first and depending on the result
execute the create.
This commit also adds missing run_once and delegate_to statement.

[1] https://github.com/ceph/ceph/commit/269e9b9

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>