Retry on unassigned ofport instead of treating it as a failure

Retry on unassigned ofport instead of treating it as a failure

Open vSwitch will return '[]' when querying an interface's ofport
when the ofport has not yet been assigned. This doesn't signal a
failure, but the get_port_ofport code was treating it as such.
This patch uses a decorator from python-retrying which has been
added as a dependency of oslo_concurrency and therefore packaged
everywhere. The call to fetch the ofport is retried until the
vsctl_timeout is reached and, on failure, INVALID_OFPORT is
returned.

The add_port function will attempt to delete the port if
INVALID_OFPORT is returned from get_port_ofport. add_port is also
extended to take optional Interface options so that the
add_tunnel_port and add_patch_port functions can reuse it instead
of just duplicating its functionality.

Closes-Bug: #1341020
Change-Id: Ifc52d8589c7aafd360893cb9c1cdcbf43b04ee2c