Revert "Revert "ovs-agent: Ensure integration bridge is created""

This reverts commit ccaad827b2bdcdcf4b538c5d5367f14cfc22c08f
that reverted e5cdad90f97d3a54a493eca19e7a3ff643426de1, which
was not the real cause of the problem the revert intended to fix.
Once this patch is back, change-id
I6381e3fee49910127c420dd2e3205c64cdb9e185 should fix the actual bug.

Change-Id: I4116bbb61aaf279d6d63a474134d47d97fe806f7