libsnapshot: Support cluster_ops in make_cow_from_ab_ota

Allows make_cow_from_ab_ota to specify cluster sizes with the
-cluster_ops flag.

Bug: 172026020
Test: make_cow_from_ab_ota -cluster_ops [n] [source] [dest folder]
Change-Id: Ia3aa8b3bb602944a9d4e9954be37afb262caf060