jenkins: support autobuild key or release key

jenkins: support autobuild key or release key

We need Jenkins to build ceph more often so we can have more testing,
but we don't want to sign these "unofficial" builds with the official
release key.

Conditionally select the release key or the autobuild key based upon the
$RELEASE environment variable. Jenkins will set this variable using a
parameter when it runs each ceph build. For formal releases, we will set
the parameter to "true".

Signed-off-by: Ken Dreyer <ken.dreyer@inktank.com>