Fail when no command provided with shell enabled in docker.

Review: https://reviews.apache.org/r/24987
Fixed MasterZooKeeperTest.LostZooKeeperCluster.

Review: https://reviews.apache.org/r/25195
Fixed a bug in the allocator which could cause filters to be
unnecessarily installed.

Review: https://reviews.apache.org/r/25207
Added instructions for getting started with CentOS.

Review: https://reviews.apache.org/r/25272