src/script: init test_stretch crush_collisions.sh

src/script: init test_stretch crush_collisions.sh

Add script to test for CRUSH retry exhaustion in stretch mode with
2 datacenters. Tests unbiased stretch rules by running multiple
iterations of PG mappings and checking for collisions that exceed
the 50-try limit.

Also add --show-retry-exhaustion flag to crushtool to detect and
report when CRUSH mapping hits the maximum retry limit.

Signed-off-by: Kamoltat (Junior) Sirivadhna <ksirivad@redhat.com>