Moving post_drain script execution into host_maintenance.py

Bugs closed: AURORA-806

Reviewed at https://reviews.apache.org/r/26431/
Add client cluster configuration docs.

Bugs closed: AURORA-846

Reviewed at https://reviews.apache.org/r/26834/
Decouple initial scheduling delay from first scheduling penalty.

Bugs closed: AURORA-868

Reviewed at https://reviews.apache.org/r/26901/
Add a message when coverage exceeds minimum.

This way we'll be sure to continually raise the minimum threshold as we exceed it.

Reviewed at https://reviews.apache.org/r/26902/
Document common problems and solutions when creating a new cluster.

Bugs closed: AURORA-840

Reviewed at https://reviews.apache.org/r/26832/
Improve error messages in client commands.

When a client command fails due to an error in an API call,
it used to log whatever messages came back in the reply from the server,
and then generate a message saying "... see log for details".

This change improves that behavior. Now, an error message is generated
saying something like "Command failed due to error reported by server",
followed by a group of indented lines containing the messages from the
server. For example, a new error message would look like:

	Server reported error restarting job west/bozo/test/hello
	    Job 'west/bozo/test/hello' not found.

Bugs closed: aurora-771

Reviewed at https://reviews.apache.org/r/26881/
Use mesos.native egg from svn.apache.org

Testing Done:
% ./src/test/sh/org/apache/aurora/e2e/test_end_to_end.sh
OK (all tests passed)

Bugs closed: AURORA-863

Reviewed at https://reviews.apache.org/r/26852/
Simplify the ResourceSlot class.