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.
Rename module robottelo.models to robottelo.orm

This renaming has been done for the sake of clarity. Module robottelo.orm
contains components for mapping Python classes to Satellite entities, and module
robottelo.entities contains complete mappings of Python classes to Satellite
entities.

Also:

* Fix imports in module robottelo.entities to accomodate the rename
* Flesh out docstrings in module robottelo.entities

Related to #916
Merge pull request #922 from Ichimonji10/ssh-tests

Better test `robottelo.common.ssh._get_connection`
Merge pull request #923 from Ichimonji10/api-test-plumbing

Rename module robottelo.models to robottelo.orm