Document all public variables in VARIABLES.md

[acch]

All variables which can be set by users should be documented in VARIABLES.md. Documentation should include

• variable name
• example value
• default value
• description of what this variable controls / is used for

I've already started documenting variables (see #301), this issue is to keep track of the progress:

• core/common
• core/precheck
• core/node
• core/cluster
• core/postcheck