Don't require /etc/init.d/snmpd on RHEL.

Updates the snmpd module so that we don't require
/etc/init.d/snmpd to start the snmpd service. This
should fix issues running this module on RHEL.

On Ubuntu we explicitly create /etc/init.d/snmpd
and already has some resource chaining in place so
that it will conditionally run before the snpmd service
starts.

Change-Id: I480c38d26c8b423f5a8d1ddc405749c03aae19b1
Reviewed-on: https://review.openstack.org/23460
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins