Neutron can now start before midonet-api.

Previously the provider_router was being fetched during __init__
of the midonet plugin inside neutron. If the mido-api was down
during that time the fetch would fail and neutron would crash.
provider_router will now be fetched on demand instead of during
initialization.

Signed-off-by: Alex Vasilyev <avasilyev@midokura.com>
Removing some unused imports from .../midonet/plugin.py

Some imports were mistakenly added in the previous merge to
plugin.py