set parallel_crawl_thread_count to 4 for integration tests
set parallel_crawl_thread_count to 4 for integration tests

Summary:
The default parallel_crawl_thread_count is `nproc`, which might be too heavyweight
for machines with many cores and might cause timeouts. Set it to 4 in tests to
make it less likely to timeout.

To do that, add a way to change the default watchman config via environment
variables.

Reviewed By: chadaustin

Differential Revision: D37468216

fbshipit-source-id: 2e1fbaca8cd229dca6b89187ca482456945b2221

facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman