Cert: Wait for root server to start using future

* Wait up to 15 seconds for gRPC root server to start
* Do not use signal port any more
* Also remove signal port usage from topshim tests

Bug: 197895596
Test: gd/cert/run
Tag: #gd-refactor
Change-Id: I97768a7f818f268ce6cc9942c6524ec5b5b93d2c