qa/cephadm: adjust host drain test to handle explicit placement warning

qa/cephadm: adjust host drain test to handle explicit placement warning

Since we're adding a warning if any host is listed explicitly
in the placement of any service when removing the host,
we need to adjust the host drain test that removes a host
without the --force flag to not have the explicit hostname
in the placement for the mon service.

Signed-off-by: Adam King <adking@redhat.com>