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>