Ignored the recovery error due to the inability to
find the latest executor run.

Review: https://reviews.apache.org/r/14001
Update Powered By page to reflect new organizations using Mesos.
Adds new documentation page to include links to videos and slides for Mesos presentations.
Fixed the broken OsTest.process and OsTest.killtree tests.

Review: https://reviews.apache.org/r/14072
Merge remote-tracking branch 'apache/master'