Refactor all CLI docstrings

This will make the CLI docs look much better
Trim down ForemanTask, test ForemanTask API

Remove the custom method `ForemanTask.read`. Make `ForemanTask` inherit from
`EntityReadMixin`. `ForemanTask.read_json` performs the same task that the old
`ForemanTask.read` method did, and no code was using the old method.

Add a test targeting bugzilla bug 1131702. The test asserts that when a
non-existent foreman task is read, an HTTP 4XX or 5XX error is received. The
test fails, so it is skipped appropriately.

Add this new test module to the API docs.
Merge pull request #1326 from Ichimonji10/bz-1131702

Trim down ForemanTask, test ForemanTask API
Merge pull request #1329 from elyezer/docstring-refactor-cli

Refactor all CLI docstrings
updated repo constants
Added test for update and delete roles

Added test for update and delete roles

Added test for update and delete roles
Merge pull request #1330 from tkolhar/test_cli

Added test for update and delete roles
Merge pull request #1334 from sghai/update-repo-constant

Updated repository url's under GPGKey tests with fake repo constants
Flesh out ComputeResource entity, test it.

When creating a new "compute resource" entity, a variable set of values must be
provided to the Foreman server. The exact set of values depends on the specified
provider. (e.g. EC2, Rackspace, etc.) Flesh out the ComputeResource entity with
logic necessary for handling this variability in requirements.

Add some API tests for the ComputeResource entity. It is currently *only*
possible to create a libvirt compute resource. All other tests fail for a
variety of reasons. For example:

* Creating a compute resource with a provider of 'Rackspace' causes Foreman to
  throw an HTTP 500 with the message "undefined method `upcase' for
  nil:NilClass".
* Creating a compute resource with a provider of 'GCE' fails because Foreman
  needs more data. However, the types of data needed are not discussed in the
  API docs.
* Creating a compute resource with several other providers causes Foreman to
  return the following error: "getaddrinfo: Name or service not known".