Document operator/GD2 node state machine

[JohnStrunk]

Describe the feature you'd like to have.
A number of features require the operator and GD2 to coordinate their actions, such as when decommissioning or upgrading a gluster node. This coordination can be handled via a state machine that is represented by a (GD2-level) metadata tag that is applied to gluster nodes. This issue is to fully document the states, allowed transitions, actors, and permissible actions in each state.

What is the value to the end user? (why is it a priority?)
Without proper coordination, the operator may cause a user's data to be destroyed or become unavailable.

How will we know we have a good solution? (acceptance criteria)

• Meaning of each state will be documented
• Allowed states and transitions will be documented
• Entities allowed to perform the transitions will be documented

Additional context
This is user by a number of features, including: #11 #13 #14