Introduce node.get <setting>

Summary:
Rename node.get — introduced in 75e80b72af3c — into node.get_all_properties.
This is a more explicit name to get the full dictionary.

Introduce node.get <key> to get one key.

So node.get/node.has/node.list becomes a set of commands to get a value.

Test Plan:

• Use in roles/core/network unit
• Use in new MySQL role defined at D1122

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1176