Consolidate the single zoekt_index and multiple zoekt_indices assignment logic

Background

In the Zoekt::SchedulingService.node_Assignment, we currently have separate logic for a single zoekt_index assignment and multiple zoekt_indices assignments. The single zoekt_index assignment uses the namespace storage size instead of each project's repository storage size, which is not very accurate. Also, the separate logic for the single and multiple zoekt_indices is increasing the complexity.

Proposal

Consolidate the two separate logics of assigning single zoekt_index and multiple zoekt_indices.