ML2 Cisco Nexus MD: VXLAN Bug - Global Config

Created vxlan_global_config variable to add/delete the VXLAN commands:
[no] feature nv overlay
[no] feature vn-segment-vlan-based
[no] interface nve 1
        source-interface loopback #

By default vlxan_global_config is set to False which means OpenStack will
not add or remove these VXLAN settings, and the administrator needs to
manage these operations manually or by  external orchestration.

NB: Later versions of N9K NX-OS have changed the order in which the
feature commands listed above must be removed. However, even changing
the order results in netconf not successfully removing the commands.
N9K engineers have been informed of this issue.

Change-Id: If8672b19f7c97a245a96adb53d4e8df4a937a5af