webadmin: Set default DC for import by tree selection

Instead of always choosing the first DC in the list as the default for
an import operation, consider the item selected in the system tree and
see if it implies a specific context for the import operation.

Change-Id: Ib0fc87737b151c6e39f2545e39516023b0851111
Bug-Url: https://bugzilla.redhat.com/1048726
Signed-off-by: Lior Vernia <lvernia@redhat.com>