ServiceSDK create function should accept another struct than service.Service

[NicolasMahe]

Related to #1141 (comment)

The ServiceSDK create function should accept another struct than service.Service.
It doesn't make sense to create a resource by passing the already created struct to the function that create it.
It makes validation harder by having to make compatibility with fields that should be field only after real creation (eg: hash).

As there is too many parameters to pass to the service.create function, a nice new struct should be created.

The function fromProtoService functions in service api should be replaced by a function that convert the proto definition to this new service.create struct.

[krhubert]

There is more option to discuss:

• stay as it is (pass service.Service)
• use gogo protobuf and get rid of service.Service and use an only protobuf definition
• pass protobuf definition CreateServiceRequest
• create a new struct to pass all args.

I've also put a low priority, as this one is not blocking us.

[ilgooz]

I think protobuf types should be only available inside the gRPC server. SDK shouldn't be aware of protobuf and gRPC. If Engine exposes other types of APIs in future, SDK shouldn't be affected by this.