Add configurable 120s timeout ovs-vsctl calls

By default, or with a secs of 0, ovs−vsctl waits forever for a response
from the database. This option limits runtime to approximately 120 seconds.
If the timeout expires, ovs−vsctl will exit with a SIGALRM signal. (A
timeout would normally happen only if the database cannot be contacted,
or if the system is overloaded.)

The user can update the timeout by setting the ovs_vsctl_timeout
configuration variable.

DocImpact

Closes-bug: 1229018

Change-Id: Ica64ae98fb6162d2119021972127a82417d5ee7a
Merge "Add configurable 120s timeout ovs-vsctl calls"