Add clearCookies boolean config value.

Previously, all test cases started with no cookies from the previous
test case.  This required some extra code to enable a test suite to
run as a continuous set of cases that shared one login.  Removing the
requirement to perform a log-in step on every case can save
significant time for some projects.

By default the feature clears all cookies to keep behavior consitent
with existing setups.
Merge pull request #973 from markkimsal/feature/clearCookies

Add clearCookies boolean config value.