Bug 666: Fix window_size overflow with Paramiko

This commit fixes integration testing around bug fixes in the sshproxy
layer that were causing a window_size overflow.

This also improves reliability around testing machine charms, by
verifying the sshd service is running inside the target machine (lxd)
before running tests.

Change-Id: I465d51521bf87b8e4b3dc5cac07c163fac836393
Signed-off-by: Adam Israel <adam.israel@canonical.com>