Cleaned up some log messages in the slave.

Review: https://reviews.apache.org/r/15951
Fixed MESOS-858: Ignore launch / kill requests from non-leading masters.

Review: https://reviews.apache.org/r/15952
Fixed Group to retry if authentication fails due to retryable errors.

See summary.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/15706
Fixed cgroups code to use processes rather than threads.

The cgroup 'tasks' file lists the threads in a cgroup.
We should use the 'cgroup.procs' file which lists processes.

See: https://issues.apache.org/jira/browse/MESOS-859

From: Ian Downes <ian.downes@gmail.com>
Review: https://reviews.apache.org/r/15954