Fix ssh proxy regression
Fix ssh proxy regression

The recent change [1] broke ssh proxy because ssh.close()
closes the transport connection as well.  Fix it by creating
proxy connections on every retries.

[1] Ida499a80be184b1af9b53d4ab27a8abbc4d59898

Closes-Bug: #1694458
Change-Id: Ic9a65d1da518a7253a1f6e4f4a909138f9e52de9

masayukig/tempest,masayukig/tempest,cisco-openstack/tempest,openstack/tempest,vedujoshi/tempest,Juniper/tempest,openstack/tempest,vedujoshi/tempest,Juniper/tempest,cisco-openstack/tempest