Add signing to Akraino deploy templates

Since Akraino projects do not follow standard release procedures, we
will simply sign the artifacts before pushing them to Nexus, just to
provide signatures showing that all artifacts come from LF infra.

Change-Id: Ia3478900acd02d419fe49c25b1902b1a3459f73d
Issue-ID: LF-Jira RELENG-2819
Signed-off-by: Eric Ball <eball@linuxfoundation.org>