[lager] Reduced message length and send pid to SYSLOG_PID= in lager_journald_backen

Summary:
A lager log looked like this in journaldctl:
  Dec 11 16:35:47 vlx075-tpo.tpip.net nonode@nohost[7835]: 2013-12-11 16:35:47.157 [info] <0.189.0> test
Now a Lager log will look like this:
  Dec 12 11:27:01 vlx075-tpo.tpip.net nonode@nohost[10541]: test
Date, time and priority were stored in the journal, so I removed them from the message entry. I added the pid in SYSLOG_PID= and removed this also from the message.

Test Plan:
Build
gen_event:add_handler(lager_event,lager_journald_backend,())
lager:log
and journalctl

Reviewers: hsychla, skattelmann, orixmann, hwinkel

Reviewed By: skattelmann

CC: druss, aschultz

Differential Revision: https://review.tpip.net/D1024