rbd-mirror: fail if the peer is not added

rbd-mirror: fail if the peer is not added

Due the 'failed_when: false' statement present in the peer task then
the playbook continues to ran even if the peer task was failing (like
incorrect remote peer format.

"stderr": "rbd: invalid spec 'admin@cluster1'"

This patch adds a task to list the peer present and add the peer only if
it's not already added. With this we don't need the failed_when statement
anymore.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1665877
Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>