DEFER - CSD05 - Explicit textual mention, and grammar support, for adding (extending) node operations

Node template operations can override operations specified in its node type, including overrides of type operations to NoOps. Can node templates also add operations not specified in their types? March 18 call consensus was that it would be good to make this explicit in the text where "overrides" are discussed, and Matt thought the grammar should explicitly allow extensions to operations.

An example of all the possibilities regarding operations and derivation:

```
tosca_definitions_version: tosca_simple_yaml_1_0_0

node_templates:
server1:
type: custom.nodes.Compute
properties:

1. Omitted
   interfaces:
   tosca.interfaces.node.lifecycle.Standard:
   create: # Override an operation defined in the type (even adds inputs)
   implementation: scripts/custom_create.sh
   inputs:
   some_input: hard_coded
   configure: scripts/custom_configure.sh # Adds an operation that is not implemented in the type
   start: null # Override the type operation with No-Op
2. The postconfigure is defined in the type and is not overridden

node_types:
custom.nodes.Compute:
derived_from: tosca.nodes.Compute
interfaces:
tosca.interfaces.node.lifecycle.Standard:
create: scripts/create.sh
start: scripts/start.sh
postconfigure: scripts/postconfigure.sh
```