cephadm: Check for pool existance for iscsi And NFS

cephadm: Check for pool existance for iscsi And NFS

Currently both iscsi and NFS require pools to be specified when they are
deployed. However, we don't actaully check these pools exist. Leading to
broken containers.

This patch uses the rados client that is part of the mgrmodule to check
that the specified pool exists. As we need to check in 2 different
daemons, a helper method:

   def _check_pool_exists(self, pool, service_name):

was added to `cephadm/module.py`.

Fixes: https://tracker.ceph.com/issues/45161
Signed-off-by: Matthew Oliver <moliver@suse.com>