[rfe] Add cleanhold state/verb to prepare for cleaning

Discussed during the Queens PTG gathering, in order to prevent the acquisition of current clean_steps for a node from invoking state changes, there was general consensus amongst the room to add a new state machine verb that would help facilitate the updating of cached clean step information and then hold the node in preparation for cleaning.

This has the side effect of meaning that the ramdisk could remain running for some time on the cleaning network.

A spec is being written in order to convey the idea/concept and identify any additional areas that may need work.