Fix wrong hiera key in ceph_rbdmirror

Fix wrong hiera key in ceph_rbdmirror

There is a typo in the bootstrap check which will lead to:
Could not find data item ceph_rbdmirror_bootstrap_short_node_name in any
Hiera data file and no default supplied at
/etc/puppet/modules/tripleo/manifests/profile/pacemaker/ceph/rbdmirror.pp

We need to be using the correct one:
$ hiera ceph_rbdmirror_short_bootstrap_node_name
overcloud-remote-0

Change-Id: Ic343e5f99e48360bdd2d2989781a4b6ca484e8fc