Allow disabling of long-lived SSH connections.

Currently, SSHPool connections are kept alive indefinitely.  This can
be good for many cases, but sometimes it is not desirable.  For
example, this causes unit tests which use SSH to not complete because
of the open connections.  This patch allows users to revert back
to paramiko's regular timeout policy.

Change-Id: Ifa96508b38d90610390c9ec3d48a7de75d02f08b
Merge "Allow disabling of long-lived SSH connections."