Description of problem:
1. When using webadmin import dialog, the option to select 3.5 host as a proxy host should be blocked.
2. When using webadmin import dialog, the option to select "any host in data center" when there are no 3.6 hosts on that DC should be blocked.

Currently, when trying to load VMware VMs using 3.5 host, the next failure message displayed on import dialog: 
"Failed to communicate with the external provider, see log for additional details"

Version-Release number of selected component (if applicable):
rhevm-3.6.3.4-0.1.el6
3.5 host:
vdsm-4.16.36-1.el7ev.x86_64
libvirt-1.2.17-13.el7_2.3.x86_64
qemu-kvm-rhev-2.3.0-31.el7_2.7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Using rhevm 3.6, Create DC with 3.5 compatibility. Under that DC create cluster with 3.5 compatibility. Add 3.5 host to that cluster.
2. Navigate to virtual machines tab --> import
3. Select 3.5 compatible DC, VMware source, set VMware details and click on "proxy host" dropbox.
4. Select one of the options listed in the dropbox and click "load" button.

Actual results:
3.5 host and "any host in data center" options are listed in dropbox.

Expected results:
- "proxy host" dropbox should not list anything since the host is 3.5 and there are no other 3.6 hosts under that DC.
- The next failure message displayed in the dialog: "Failed to communicate with the external provider, see log for additional details"
- The next ERROR logged in engine.log: 
"2016-03-14 14:46:17,517 ERROR [org.ovirt.engine.core.bll.GetVmsFromExternalProviderQuery] (ajp-/127.0.0.1:8702-16) [] Exception: org.ovirt.engine.core.common.errors.EngineException: EngineException: org.ovirt.engine.core.vdsbroker.vdsbrok
er.VDSErrorException: VDSGenericException: VDSErrorException: Failed to GetVmsFromExternalProviderVDS, error = The method does not exist / is not available., code = -32601 (Failed with error unexpected and code 16)
"

Additional info:
Screenshots and engine log attached.