When retrieving a vacant L3 agent binding index, if
"is_manual_scheduling" is set, the method "get_vacant_binding_index"
should always return a valid binding index. If the existing binding
indexes are sequentially aligned, the method will return a new one
on top; if there is a gap in the binding indexes list, the first
free index will be returned.
Closes-Bug: #1884906
Change-Id: I0a89bca0734d3e735fb357e488f85589e81d709f