ceph ansible 3.1 igw: fix rbd-target-gw startup

ceph ansible 3.1 igw: fix rbd-target-gw startup

The problem is rbd-target-gw needs the rbd pool to be created, keyring
to be copied over, and the iscsi-gateway.cfg to be setup before starting
the rbd-target-gw service.

In the master branch this is fixed by this commit:

    commit 91bf53ee932a6748c464bea762f8fb6f07f11347
    Author: Sébastien Han <seb@redhat.com>
    Date:   Fri Mar 23 11:24:56 2018 +0800

        ceph-iscsi: support for containerize deployment

where the needed setup tasks are done in common.yml which is done
before prerequisites.yml.

To avoid porting all those changes to 3.1 this patch just moves the
rbd-target-gw startup to configure_iscsi.yml after everything has
been setup.

This fixes red hat bz:

https://bugzilla.redhat.com/show_bug.cgi?id=1601325

Signed-off-by: Mike Christie <mchristi@redhat.com>