1124570 - Adds two-queue implementation for reserved resources

https://bugzilla.redhat.com/show_bug.cgi?id=1124570

This BZ implements this [design and TODO list](https://fedorahosted.org/pulp/wiki/TwoQueueReservedResourceDesign). Refer to that doc for more info on what is contained in this PR.
I went with a raise style exception for these. It makes the return arguments cleaner, and it's better this way. I added sphinx docs on the raise types and situations. Other code and tests were updated accordingly.