Fix broken method call
Do not instantiate module-wide SSH connection

Do not instantiate a module-wide SSH connection when module
robottelo.common.ssh is imported. Instead, force code to create SSH connections
as needed.
Add unit test for module robottelo.common.ssh

Add a simple test for method `_get_connection`.
Merge pull request #900 from Ichimonji10/ssh

do not create SSH connection on import