qa/cephadm: add test for ca signed keys

qa/cephadm: add test for ca signed keys

Test that bootstraps with a CA signed key using
the use_ca_signed_key cephadm override. Then follows
up by doing a check-host on each host which verifies
the cephadm mgr module can reach and authenticate with
the nodes using the new key setup.

This probably should really be a workunit, but
I didn't want to create a full new section for
this test and I needed a section that didn't
already run the cephadm task for every test. I could
see this being moved into some sort of
"test_special_deployment_scenarios" section in the future

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