nrwt: make the delay between starting workers configurable per-port

nrwt: make the delay between starting workers configurable per-port
https://bugs.webkit.org/show_bug.cgi?id=79148

Reviewed by Eric Seidel.

Because of bug 79147, we have to sleep a bit in between starting
DRTs; however, doing so across the board slows down the Test
port. Making this configurable shaves 10 seconds off of
run_webkit_tests_integrationtest.py

* Scripts/webkitpy/layout_tests/controllers/manager.py:
(Manager._run_tests):
* Scripts/webkitpy/layout_tests/port/base.py:
(Port.worker_startup_delay_secs):
* Scripts/webkitpy/layout_tests/port/test.py:
(TestPort.worker_startup_delay_secs):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108413 268f45cc-cd09-0410-ab3c-d52691b4dbfc