mgr/cephadm: continue in nfs service purge if grace file is already deleted

mgr/cephadm: continue in nfs service purge if grace file is already deleted

The test_nfs task we run in teuthology creates and removes a number of
nfs clusters during the task. I think it's possible based on timing for
it to end up in a situation where it tries to remove an nfs service before
the grace file has been created. In that case, cephadm doesn't know it
hasn't created the grace file and just repeatedly fails forever attempting
to remove the nonexistent file. This patch adds handling for the error
case where we get a nonzero rc but the error message implies the command
failed because the file already does not exist.

Fixes: https://tracker.ceph.com/issues/69736
Signed-off-by: Adam King <adking@redhat.com>