Add unit tests for midonet_lib.py

Add unit tests for ChainManager, PortGroupManager and
RuleManager in midonet_lib.py.

Signed-off-by: Ryu Ishimoto <ryu@midokura.com>
Remove whitespace from test_midonet_plugin.py

Remove unnecessary whitespaces from test_midonet_plugin.py to
pass pep8.

Signed-off-by: Ryu Ishimoto <ryu@midokura.com>