CLOUDSTACK-3759 [Automation] Failed to attach volume to VM, while vm are in stopped state Fix the null pointer.

Signed-off-by: Edison Su <sudison@gmail.com>
Automation: Reconnect the ssh for test_vpc_vm_life_cycle.py

Since VM are reused for test cases, we need to reconnect the SSH client to avoid
error like:

Failed to SSH into VM, SSH session not active
Automation: fixed cleanup issue with test_vpn_user.

Reviewed-by: Sheng Yang <sheng.yang@citrix.com>
Signed-off-by: Sheng Yang <sheng.yang@citrix.com>