Make dvr_vmarp_table_update call conditional to dvr extension

Without making this call conditional, every l3plugin that
integrates with the ML2 plugin will need to implement this
method and this must not be necessary.

Closes-bug: #1349638

Change-Id: Ie9ba3bad4152810f5bfa530be54be70139cebc0c
Merge "Make dvr_vmarp_table_update call conditional to dvr extension"