Support for configurable Mesos role

[Zhitao Li <zh...@gmail.com>]

Hi,

I have been working under Will Farner's help on a fix for AURORA-1109
<https://issues.apache.org/jira/browse/AURORA-1109>, which added support to
configure (an optional) Mesos role which Aurora will be running under. This
is useful for operators who run Aurora with other Mesos framework(s) in a
shared cluster and need to reserve resource for Aurora.

For board visibility in case anyone missed it or really interested, please
check out the review at https://reviews.apache.org/r/42126/, which also
contains some discussions on how this is implemented.

Please feel free to comment on the JIRA issue, the review or email me with
any question/feedback.

Thanks!

-- 
Cheers,

Zhitao Li