dist/debian: install GPG key for cross-building
dist/debian: install GPG key for cross-building

We found on some Debian environment Ubuntu .deb build fails with
gpg error because lack of Ubuntu GPG key, so we need to install it before
start pbuilder.
Same as on Ubuntu, it needs to install Debian GPG key.

See scylladb/scylla#3823

Signed-off-by: Takuya ASADA <4ebfb6f510af412d6e89fefa3ab96d9c8b5e3fff@scylladb.com>
Message-Id: <20181008110724.18335-1-4ebfb6f510af412d6e89fefa3ab96d9c8b5e3fff@scylladb.com>

scylladb/scylla-jmx,scylladb/scylla-jmx,scylladb/scylla-jmx