Update gentoo-syslog consumers to use tail-syslog

Update gentoo-syslog consumers to use tail-syslog

Folks who just used the stock:

  CMD rc default && tail -F /var/log/messages

can now piggy-back on the CMD setup in the gentoo-syslog
Dockerfile.template, so I removed their CMD lines.

I updated folks that did fancier things (e.g. tweaking configs before
'rc default') to use 'exec tail-syslog' instead of 'tail -F
/var/log/messages'.

Folks that are built FROM gentoo-syslog but never used 'rc default'
should probably be FROM a more basic image, but I left them alone in
this commit.