Added UI CRUD functions and simple tests for manifest

1) Added UI CRUD functions for manifests
2) Added simple tests for manifests
3) We are now able to generate, download manifests and upload them to the server even with WEB-UI.

4) The manifest being created here have almost 7 different products with different subscription ID's and it takes time to create such manifest. 
(robottelo)[kedar@XXX robottelo]$ python -m unittest tests.foreman.ui.test_subscription
..
----------------------------------------------------------------------
Ran 2 tests in 719.544s

OK

5) To speed up may be we can decide upon the basic subscriptions/SKU's  to be added to the account. :)