Rsyslog upgrade

Fixes #57322202 in the CF Kibana backlog. This upgrades rsyslog in ubuntu and centos stemcells to version 7.4.6, which supports RELP: http://www.rsyslog.com/doc/relp.html

We successfully built aws ubuntu, vsphere ubuntu, and vsphere centos stemcells, and deployed the aws ubuntu stemcell to our AWS CF environment. We also verified that RELP was working between the nats job and syslog aggregator job.