mgr/cephadm: allow setting insecure_skip_verify for alertmanager

mgr/cephadm: allow setting insecure_skip_verify for alertmanager

Add a "secure" parameter to alertmanager spec that will cause it
to deploy alertmanagers with insecure_skip_verify as true or false
depending on the value given for "secure".

NOTE: alertmanager must still be reconfigured after applying a yaml
with this option changed.

Fixes: https://tracker.ceph.com/issues/55272
Fixes: https://tracker.ceph.com/issues/55333
Signed-off-by: Adam King <adking@redhat.com>