engine: Change Variable Name

This patch changes a variable name from networksInVdsByName
to networksInClusterByName, according to what the list
represents.

Change-Id: I224265aeb35aeba22832a91e79b6ca8e66d01b1e
Bug-Url: https://bugzilla.redhat.com/873338
Signed-off-by: Muli Salem <msalem@redhat.com>