Add support to dynamically upload drivers in PLUMgrid plugin

PLUMgrid plugin will be able to dynamycally upload any of its available drivers.
It will simplify any CI testing by changing the configuration file for the plugin
instead of changing the code directly.

Change-Id: I56da881688cfdf8f9a1b655c1080c39ffc0133a5
Closes-bug: #1315137
Ensure core plugin deallocation after every test

The unit tests were previously consuming an excessive amount of memory
(4GB+) due to plugin instances persisting in memory.  Deallocation was
not possible where a combination of circular references and mocking
was involved.  This patch ensures that only NeutronManager holds a
plugin reference and that all other references are instances of
weakref.proxy.  Residual memory footprint for tox executed on a
12-core machine has been reduced to ~1.3GB.  Plugin deallocation is
validated at the end of each test to prevent regressions.

This change also includes fixes to unit tests that depended on plugin
instances persisting across tests.

Partial-Bug: #1234857
Change-Id: Ia1f868c2d206eb72ef77d290d054f3c48ab58c94
Merge "Ensure core plugin deallocation after every test"
Merge "Add support to dynamically upload drivers in PLUMgrid plugin"