examples/ct: public key is optional for server
examples/ct: public key is optional for server

Fixes #555

gdbelvin/trillian,daviddrysdale/trillian,RJPercival/trillian,gtank/trillian,Martin2112/trillian,daviddrysdale/trillian,daviddrysdale/trillian,AlCutter/trillian,codingllama/trillian,google/trillian,Martin2112/trillian,gdbelvin/trillian,codingllama/trillian,google/trillian,RJPercival/trillian,gdbelvin/trillian,AlCutter/trillian,gtank/trillian,Martin2112/trillian