Drop booby, let nil values be submitted to API

Drop dependency on booby.

* Make class `Entity` inherit directly from `object`. Define method
  `Entity.__init__`, and make that method handle whatever instantiation logic
  booby formerly did. (but better!)
* Make class `Field` inherit directly from `object`. Define method
  `Field.__init__`, and make that method capable of recording the instance
  attributes `required`, `choices`, `default` and `null`.

This has several positive effects. First, fewer methods are necessary.

* Drop method `robottelo.factory.field_is_required`. The `required` attribute is
  guaranteed to exist on all `Field` classes, so it is possible to just call
  `SomeField.required` instead.
* Drop method `robottelo.orm.Entity.get_values`. It is now possible to just call
  `vars(SomeEntity)` instead.

Second, all classes inheriting from `Entity` and `Field` have a more
comprehensible inheritance tree and sane instantiation process. An `update`
method is no longer inherited from booby, which means that an
`EntityUpdateMixin` may finally be created without MRO issues, and no metaclass
magic is used. Incidentally, a pylint error stating that many `Entity`
subclasses have too many ancestors is now gone.

Third, it is now possible to pass `None` values into classes. This fixes #1082.
Formerly, this behaviour was exhibited:

    >>> from robottelo.entities import Organization
    >>> for org in (
    ...         Organization(name='ACME'),
    ...         Organization(name='ACME', description=None),
    ...         Organization(name='ACME', description='Better than ACNE!')
    ... ):
    ...     org.get_values()
    ...
    {'name': 'ACME'}
    {'name': 'ACME'}
    {'description': 'Better than ACNE!', 'name': 'ACME'}

This is the new behaviour:

    >>> from robottelo.entities import Organization
    >>> for org in (
    ...         Organization(name='ACME'),
    ...         Organization(name='ACME', description=None),
    ...         Organization(name='ACME', description='Better than ACNE!')
    ... ):
    ...     vars(org)
    ...
    {'name': 'ACME'}
    {'description': None, 'name': 'ACME'}
    {'description': 'Better than ACNE!', 'name': 'ACME'}

Drop a `pylint:disable` directive from module `robottelo.entities`. All classes
inheriting from `Entity` now have an `__init__` method.

These changes do not appear to break anything. Module `test_multiple_paths` is
in good shape:

    $ python -m unittest tests.foreman.api.test_multiple_paths
    …
    ----------------------------------------------------------------------
    Ran 238 tests in 154.352s

    FAILED (failures=1, skipped=62)

The one failed test relates to activation keys, which is a known issue. `make
test-robottelo` also succeeds.
Merge pull request #1615 from Ichimonji10/experiment

Drop booby, let nil values be submitted to API
Fix/Update remaining CLI users tests

Email validation have change on foreman and all tests that checks for
email needed to be updated.

Fix test_negative_delete_user_1 to only call the factory if the login is
not provided in the test data.
Skip and improve some CLI content-view tests

Skip tests that uses the hammer content-view info output because its not
consistent with the rename of environments to
lifecycle-environments.

Also improve assertion for test_cv_promote_custom_content and
test_cv_promote_composite

Bonus: remove skip_if_bug_open for the verified BZ 1127408 related to
the repository info failure
Merge pull request #1620 from elyezer/cli-user-tests

Fix/Update remaining CLI users tests
Merge pull request #1625 from elyezer/cli-content-view

Skip and improve some CLI content-view tests
Test:Check if multiple Activation keys can be attached to a system
- added a function to fetch the associated content-host with activation-key
- updated top menu locator names
Merge pull request #1627 from sghai/ak-test

Test:Check if multiple Activation keys can be attached to a system
removed unwanted navigation for some tests.

The navigation would be taken care by the search function, which
is called upon by the update function.
Merge pull request #1628 from kbidarkar/fix-os-ui

removed unwanted navigation for some tests.
Added Default org and loc selection support to users.

We now have support for `default_org` and `default_loc`,
selection under the org and loc tabs for the users.
Merge pull request #1629 from kbidarkar/user-default-org

Added Default org and loc selection support to users.
Test to check if the activation-key's content-host limit actually limits its usage
Merge pull request #1630 from sghai/ak-limit-test

Test to check if the activation-key's content-host limit actually limits its usage
Updated all instances of  katello-ca rpm version for client side
Merge pull request #1633 from sghai/fix-rpm-ver

Updated all instances of  katello-ca rpm version for client side
Added test - to check content host names are unique
Added clone function and test for cloning provisioning template.

Few docstrings updates.
Rewrite ContentViewTestCase

* Drop an unnecessary `@ddt` decorator.
* Make the test code more straightforward.

Rather magically, the test now succeeds.

    $ nosetests tests/foreman/api/test_contentview.py:ContentViewTestCase
    .
    ----------------------------------------------------------------------
    Ran 1 test in 19.436s

    OK

This fixes a Jenkins failure.
Merge pull request #1609 from tkolhar/test_cli

Added test - to check content host names are unique