Reorganize Manila scheduler and merge in code from Oslo incubator

With oslo-incubator going away, we need to pull those classes into the Manila code base, along with their unit tests. This presents a good opportunity to do some long-needed housecleaning. This blueprint will accomplish the following:

    1. Move the scheduler classes from manila.openstack.common.scheduler to manila.scheduler.
    2. Add the unit tests from olso-incubator into Manila.
    3. Remove duplication among the combined scheduler modules.
    4. Move scheduler drivers into a sub-module.
    5. Normalize class and module naming throughout the scheduler.
    6. Split some unit test files so they match the names of the modules that they test.
    7. Convert usage of mox & oslotest to mock & unittest.
    8. Add a few unit tests to boost coverage levels