Break GdDevice into GdDevice and GdCertDevice

We need distinct devices, since the gRPC capabilities are
different from each other and we need to tell them apart
from each other in tests.

Also, remove unused gd/cert/run_cert.py
Also, don't indicate rootcanal error at end of test if
return code indicates it was terminated by the SIGINT we sent it.
(-N return code indicates terminated by N signal, as per
https://docs.python.org/2/library/subprocess.html#subprocess.Popen.returncode)

Test: cert/run_cert.sh
Change-Id: Idf14a05d73f2f80f5b871b2c41118219c27a0f76