Support optional alternate OOB power operations hook

Provide a hook for executing a custom module instead of performing OOB power operations

In our environment, in-band and out-of-band traffic are on separate networks, so our admin node does not have direct access to EKSA node BMCs. For our standard bare-metal builds, we run an agent on a dual-homed host) that queries a central service for OOB tasks (power mgmt and the like), and then performs those tasks directly. If Tinkerbell could optionally execute a module of our choosing as part of the provisioning process (instead of attempting power operations itself), we could make a REST call to our central service and bypass many/most of the issues concerned with traversing those two networks. This would also improve the security posture of hardware.csv.