[1LP][RFR] Add multiple_iso_datastore test and minor improvements

[spusateri]

Purpose or Intent

Adding tests/Enhancement: Added test for the provisioning of multiple iso datastores with 2 providers. The test assures that a secondary datastore can be added as long as there is at least 2 distinct providers. Additional markers were introduced to replace the outdated 'pytest_generate_tests()' method.

PRT Run

{{pytest: cfme/tests/infrastructure/test_iso_provisioning.py -k "test_add_multiple_iso_datastore"}}

[jawatts]

There is a manual test case defined here: https://github.com/ManageIQ/integration_tests/blob/master/cfme/tests/cloud_infra_common/test_provisioning_manual.py#L383

Please remove it from the repo in this PR