Set slave subsystems only in containerizer tests and cleaned up the
code using cgroups::enabled().

Review: https://reviews.apache.org/r/21585
Included the error message when nl_connect fails.

Review: https://reviews.apache.org/r/21595
Added elected time to master's web ui.

Review: https://reviews.apache.org/r/21501
Fixed the broken build on OSX caused by cgroups::enabled().

Review: https://reviews.apache.org/r/21664