Make scheduler disk_filter take swap into account

The disk_filter, which makes sure a node has enough disk space for an
instance doesn't take the swap partition into account, but swap takes up
disk space so it should.

Change-Id: I0779dcf0f1cd60017b5b49ca38c0a15b23d802d7
Closes-Bug: #1239009
Merge "Make scheduler disk_filter take swap into account"