Added native implementation OVSDB API. Both APIs may be enabled
via configuration file. The default one is the CLI vsctl.
A new configuration variable, ``ovsdb_connection``, is added to
define the connection string for the OVSDB backend.
Added functional tests to vif_plug_ovs. This commit also includes
the base functions to execute functional tests and a set of them
to test the OVSDB APIs: native and ovs-vsctl.
Closes-Bug: #1666917
Change-Id: I86fbf8c67572e51889eb091d7bff7f9350b52481
1546d349b1