Introduce oom plugin and disable syslog plugin

The oom plugin is improves handling of out-of-memory conditions
in gears by dynamically adjusting a cgroup's memory limit while
cleaning up its tasks.
Remove an incorrect comment line in oom_plugin
Disable OOM kills for gear cgroups
Add OOM_CHECK_PERIOD to oo-watchman man page
Merge pull request #5429 from a13m/oom-plugin

Merged by openshift-bot