cephadm: use gpg key for add-repo on ubuntu/debian

cephadm: use gpg key for add-repo on ubuntu/debian

We were using the ascii version of the gpg key which
was marked as an unsupported filetype by apt-get which
caused apt-get to not make use of the repo source we
were adding.

Additionally, added aomething to make sure we update the
package list after adding the source and key

Fixes: https://tracker.ceph.com/issues/44972
Fixes: https://tracker.ceph.com/issues/45009
Signed-off-by: Adam King <adking@redhat.com>
(cherry picked from commit 54055381fd61d290bf3b44ec01e7d4f3a0eb5f1c)