Add new simplified, single-line list notation / grammar for Requirement Def.

The current grammar for Requirement Def. is as follows:

<requirement_name>:
node: <node_type_or_template_name>
capability: <capability_type_or_template_name>
relationship: <relationship_type_or_template_name>

The new notation would allow the a single-line, list notation for a very common case where only the 'tuple names (i.e., node, capability, name are needed (no target_filter) as follows:

requirement: [ <node_name>, <capability_name>, <relationship_name> ]

For example, the new notation would allow the following host requirement:
requirements:

• db_admin_console:
  node: my_database
  capability: db_admin_endpoint
  relationship: my_connection

to be written more simple as:

requirements:

• db_admin_console: [ my_database, db_admin_endpoint, my_connection ]

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Note: we may want to discuss ordering of list to put relationship in the 2nd entry in the list instead of the capability, whatever seems to be the most common use case (i.e., are relationships or capabilities needed more often when describing a requirement?).