Add remote signing and consolidate remote cert generation

* CFSSL now accepts a "remote" parameter in the signature and new certificate requests. This remote parameter may specify a remote CFSSL instance to perform the signature operation. In the case of new certificate generation, the private key remains on the originating CFSSL instance.

* The "remotecert" endpoint has been removed in favour of the following approach.

* The remote argument now applies to the serve command. If supplied, both the signing API and certificate generation endpoints will automatically ship out signature requests to this remote without clients having to supply a "remote" parameter. Note that the "remote" parameter overrides this option.