Currently, if an IsoDatastore is defined for any EMS of the same type, the Add a New ISO Datastore button will be disabled. For example, if a user has 2 Red Hat EMSes they will only be to create an IsoDatastore on one of them. This code changes the logic for disabling the button to look for any EMSes without datastores before disabling the button. Also moves a couple of queries into convenience methods on the model, and adds a couple of tests.
https://github.com/ManageIQ/manageiq/pull/10046
Verified on CFME-5.7.0.3.20160927165516_075d0f3. Possible to add a New ISO datastore from RHV-4.0.4, though there is already ISO datastore added from RHV-3.6.8-0.1.el6