mgr/dashboard,prometheus: fix handling of server_addr

mgr/dashboard,prometheus: fix handling of server_addr

Commit 157a7b4 corrected the URI published by dashboard/prometheus
to use get_mgr_ip() if the address was the wildcard (eg ::).
However, the change also affected the cherrypy server.socket_host
config value, so the modules could no longer bind to a wildcard.

This patch corrects the commit to not affect the cherrypy config.

It also further corrects the dashboard URI fix to handle the 0.0.0.0
wildcard case (prometheus already had this behavior).

Fixes: 157a7b4183dbd888f106c613a758409d7e07b917
Fixes: http://tracker.ceph.com/issues/52002
Signed-off-by: Scott Shambarger <devel@shambarger.net>