engine: Improve iSCSI Bond CDA validation

Add following CDA validations due to adding iSCSI Multipathing support
through the REST API:

1. Validate that datacenter version is at least 3.4.
2. Validate that added logical networks belong to the same datacenter as
the specified iSCSI Bond and that none of them is "required" network.
3. Validate that added storage targets are indeed iSCSI targets, they
are "connectable" and they belong to the storage domain from the same
datacenter as the specified iSCSI Bond.

Change-Id: I1d0ac7e6af6cb87b500740d679af2f13e63dbeb5
Bug-Url: https://bugzilla.redhat.com/1108174
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>
Signed-off-by: Tal Nisan <tnisan@redhat.com>