Assign existing unused VLANs when possible

[sajith]

Grabbing an issue for this TODO item:

pce/src/sdx/pce/topology/temanager.py

Lines 580 to 593 in 1e19555

	def _find_vlan_on_path(self, path):
	"""
	Find an unused available VLAN on path.
	Finds a VLAN that's not being used at the moment on a provided
	path. Returns an available VLAN if possible, None if none are
	available on the submitted path.
	output: vlan_tag string or None
	"""
	# TODO: implement this
	assert False, "Not implemented"