Relax the VLAN continuity requirement on dataplane path computation

[YufengXin]

    See the code at sdxctlr/TopologyManager.py

line 487.
def find_vlan_on_path(self, 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.
'''
self.dlogger.debug("find_vlan_on_path: %s" % path)
selected_vlan = None
with self.topolock:
for vlan in range(1,4089):

[YufengXin]

However, this also requires implementation of vlan translation flow rules in the intermediate switches.