Make timeout for ovs-vsctl configurable

This patch adds a new configuration variable for the timeout on
ovs-vsctl commands, and sets the default timeout to 10 seconds.
This is aimed at allowing users to tune the agents in order to avoid
timeout errors on their deployments.

Change-Id: I73ea0d0de49a4b4a118bc2d68ad9c093ea122717
Closes-Bug: #1254520

Implements: DE1240
Upstream-Review: https://review.openstack.org/#/c/61105/