Renamed tests.common.MgmtSystem to mgmt_system

I've got a branch that adds an EC2 Management system, and also PEP8's the module name to be mgmt_system.

It seemed like a good idea to break that up into two separate pulls, so this is the one where the tests.common.MgmtSystem gets renamed to tests.common.mgmt_system, and references to it are updated. There are also some minor changes like trailing whitespace removal, docstring wrapping, etc. more in-line with PEP8 style.