qa/standalone/scrub: add build_pg_dicts()

qa/standalone/scrub: add build_pg_dicts()

a helper function that builds bash dictionaries:
pg to acting set, pg to primary & pg to pool.

Also added are two helper functions that make use of the dictionaries:

count_common_active() to count the number of common OSDs
in the acting set of two PGs, and find_disjoint_but_primary()
to find a PG that is disjoint from the first PG, apart from
possibly having the same primary OSD.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>