arcyd-tester: expose '--repo-count' parameter

Make it possible to choose the number of repos to test from the
command-line.

Test Plan:
$ ./precommit.sh
$ ./proto/arcyd-tester --repo-count 2
.. two repos are tested ..
Revert "TEMP"

This reverts commit 670dd4652c82bc7fb2d984154c81cd89bb752f6f.

Conflicts:
	py/ate/atecmd_arcydtester.py