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
Create some simple tests for path api/v2/hosts

Implement a few tests which check whether it's possible to GET the path, and add
some partially written tests for doing other things like POSTing and messing
with sub-paths of api/v2/hosts.
Use new `requests` wrapper
Add a new API test

Ensure that a HTTP 401 response is generated when an unauthorized HTTP GET
request is made.