Midonet plugin decomposition

Midonet plugin decomposition

Remove the midonet plugin code and add the requirements.txt file to
set the dependency to the vendor plugin. Dependency is not pinned: we
will use the newest library during the Kilo cycle and we will submit a
patch with the pinned version a bit before the Kilo code freeze.

Vendor plugin code is available here:
https://github.com/midonet/python-neutron-plugin-midonet

Plugin already available in pypi:
https://pypi.python.org/pypi/neutron-plugin-midonet/

DocImpact
Partially-implements: blueprint core-vendor-decomposition
Closes-bug: #1408339

Change-Id: I867ab6753cd45abb65e850cd6aaef7bb0c5bbddb