Allow health checkers to abort if the request context is canceled.
Modify the checkers to respect context cancelation and return wrapped
errors so the caller of CheckStatus() would be able to discriminate true
failed checks from checks which were aborted because the context became
done.
Signed-off-by: Cory Snider <csnider@mirantis.com>
Also, add timeout and status code parameters to the HTTP checker, and
remove the threshold parameter for the file checker.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
