Parse dhchap_host_key on controller level

Parse dhchap_host_key on controller level

The json config schema declares an 'dhchap_key' element representing
the dhchap host key on the controller level, but the implementation
was missing. Add the missing parsing elements and ensure to pick the
correct one (either host or controller setting) when creating the
nvme connect string.

Signed-off-by: Hannes Reinecke <hare@suse.de>