Better test `robottelo.common.ssh._get_connection`

Do not actually attempt to create SSH connections when testing
`_get_connection`. Instead, mock up `paramiko.SSHClient` and intercept calls to
methods like `connect` and `close`.
Make pylint happy

Most of pylint's warnings were spurious, but at least one set of warning was
valid: the attribute-defined-outside-init warning.
Merge pull request #922 from Ichimonji10/ssh-tests

Better test `robottelo.common.ssh._get_connection`