do not create SSH connection on import

Prevent an SSH connection from automatically being established when module `robottelo.common.ssh` is imported. Instead, generate connections as needed. __This may significantly reduce the speed of the test suite.__

Automatically close SSH connections, just like one would do with a file handle.

Fix a broken method call.
As `_get_app_root` will be used by other modules it should be renamed to `get_app_root`