Norm. Cap. Type "Endpoint.protocol" missing required=true in schema defn.

The definition in the TOSCA v1.0 Simple Profile for "Endpoint" Capability type has "protocol" as being a required property; however the schema does not have the key "required" at all. That is OK since it should default to true (per spec. rules for property definitions), but the schema should be explicit where possible to avoid ambiguities.

The schema today reads:
tosca.capabilities.Endpoint:
derived_from: tosca.capabilities.Root
properties:
protocol:
type: string
default: tcp

we should add "required: true".

It is also interesting that this is a rare required property that also has a "default" in that even if a template does not include "protocol" as a required property it should not be an error since it has a "default" value of "tcp" supplied by schema.