mgr/cephadm: add ability for cephadm to create self-signed cert for RGW

mgr/cephadm: add ability for cephadm to create self-signed cert for RGW

If users are okay with a self-signed cert and would prefer to
just let cephadm generate it for them rather than creating it
themselves. Additionally, if the zonegroup_hostnames field is
set, add those entries to the SANs entries for the cert in
order to facilitate virtual host bucket access

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