QA: use a persistent SSH connection to the master

The recent additions to QA (many more tests) make QA slow if the
machine on which the QA runs is not very close to the tested nodes —
or in general, when the SSH handhaske is costly.

We discussed before about using a persistent connection, and here is
the patch that implements it. On a very small QA (very very small), it
cuts down a lot of time (almost half), so it should be useful even for
a full QA.

I've also thought about changing from external ssh to paramiko, but I
estimated that it would be more work to correctly interleave the IO
from the remote process than just running a background SSH.

Also note that yes, the global dict is ugly, but I don't know of
another simple way to implement this.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>