Adds RHEL support to snmpd module.

Parameterize the package name so that things work on multiple
distros.

Also, updates the snmpd module so that that the init script is
written out on Ubuntu only (this is not required on RHEL).

Change-Id: Id5868b85fc0a7a6057d9be16c76dad2efb834f0e
Reviewed-on: https://review.openstack.org/23186
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins