Update requirements.txt for ML2 Nexus
Update requirements.txt for ML2 Nexus

The version number to the lxml library is not correct.
The ML2 Nexus driver requires version 3.3.3 NOT 0.3.3

Change-Id: I25d21b9b470517caaffce44561b85bdb0c4e8649
Closes-bug:  #1486164

CiscoSystems/networking-cisco,noironetworks/networking-cisco,Gitweijie/first_project,Tehsmash/networking-cisco,Gitweijie/first_project,CiscoSystems/networking-cisco,openstack/networking-cisco,Tehsmash/networking-cisco,noironetworks/networking-cisco,openstack/networking-cisco