Make http signaler timeout secs configurable via environment variable.

This is a band-aid for the issue described in AURORA-224. Further
improvements would be made possible by refactoring the updater
to completely remove client-side health checking (delegating to
thermos, which already supports a much richer system of delegated
health checks).

Testing Done:
./build-support/jenkins/build.sh

Bugs closed: AURORA-224

Reviewed at https://reviews.apache.org/r/18602/