Re-deprecated 'webui_hostname' as optional (from required).

We still need to set the 'webui_hostname' field for now in order for
interoperating with code that expects it to be required (e.g., a slave
on an older release). After another deprecation cycle we can either
remove this field entirely or just keep it around as an artifact so
that we don't reuse the ID.

Review: https://reviews.apache.org/r/15214
Fixed the cgroup isolator RSS memory reporting.

Report the number of bytes used by the memory cgroup, and stop
reporting memory with no file backed pages taken into account.

From: Eric Biederman <ebiederm@xmission.com>
Review: https://reviews.apache.org/r/14825
Fixed configure.ac to use $CC instead of 'gcc' for version checking.

This has caused problems in settings where the desired compiler
isn't the system bundled GCC (as in Mac OS X Mavericks builds where
we have been using specific versions of GCC).

Review #14164 is re-enabled for $(CC) instead of 'gcc' and lets us
compile with GCC 4.8 on Mavericks instead of down-grading to 4.7.

From: Niklas Nielsen <nik@qni.dk>
Review: https://reviews.apache.org/r/15193
Preferred ng-show to jQuery for #no-leader-alert.

Mixing jQuery DOM access with Angular creates two locations for state
storage. Always prefer Angular to jQuery for changing the DOM.

* Removed the "Close" button because the alert is not dismissable.

Review: https://reviews.apache.org/r/15154
Fixed a race condition in the Cgroups Isolator.
Removed an incorrect TODO.

Review: https://reviews.apache.org/r/15044
Fixed a pid comparison in the scheduler driver.

Review: https://reviews.apache.org/r/15065