Check for presence of 'X-USER-TOKEN' in self._headers before deleting
Check for presence of 'X-USER-TOKEN' in self._headers before deleting

self._headers can be set to empty if self._authenticate raises an
Exception and hence we need to check if key 'X-USER-TOKEN' is
present before trying to delete it

Change-Id: I31bba531667f424de30c0c03302301c0c171a60d
Partial-Bug: #1616797
(cherry picked from commit 681872321c672a4d581211bbda418e1b0d474983)

tcpcloud/contrail-controller,tcpcloud/contrail-controller,tcpcloud/contrail-controller,tcpcloud/contrail-controller,tcpcloud/contrail-controller,tcpcloud/contrail-controller