ovs-vsctl: Try connecting only once for active connections by default.

ovs-vsctl: Try connecting only once for active connections by default.

Until now, ovs-vsctl has kept trying to the database server until it
succeeded or the timeout expired (if one was specified with --timeout).
This meant that if ovsdb-server wasn't running, then ovs-vsctl would hang.
The result was that almost every ovs-vsctl invocation in scripts specified
a timeout on the off-chance that the database server might not be running.
But it's difficult to choose a good timeout.  A timeout that is too short
can cause spurious failures.  A timeout that is too long causes long delays
if the server really isn't running.

This commit should alleviate this problem.  It changes ovs-vsctl's behavior
so that, if it fails to connect to the server, it exits unsuccessfully.
This makes --timeout obsolete for the purpose of avoiding a hang if the
database server isn't running.  (--timeout is still useful to avoid a hang
if ovsdb-server is running but ovs-vswitchd is not, for ovs-vsctl commands
that modify the database.  --no-wait also avoids that issue.)

Bug #2393.
Bug #15594.
Reported-by: Jeff Merrick <jmerrick@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>