Test plugin should set SSH_AGENT_PID and SSH_AGENT_SOCK if declared

Otherwise, when you need to branch charms that you are developing/testing, you get prompted for your key passphrase.

2014-01-22 20:17:04 Starting deployment of cstack2
Enter passphrase for key '/home/dpb/.ssh/id_rsa':
Enter passphrase for key '/home/dpb/.ssh/id_rsa':
Enter passphrase for key '/home/dpb/.ssh/id_rsa':
Enter passphrase for key '/home/dpb/.ssh/id_rsa':
Enter passphrase for key '/home/dpb/.ssh/id_rsa':

After some testing, I have just inserted the following:

 class Conductor(object):
     def __init__(self, arguments=None):
         self.args = arguments
         self.env = {'JUJU_HOME': os.path.expanduser('~/.juju')}
+ self.env.update(os.environ)

Which seems to be working fine. In thinking things out, passing env variables down to the script seems to be pretty necessary for any kind of test framework.