Discover nodes, knowing IP range for their BMCs and the default IPMI credentials

*** Create a new action tripleo.baremetal.validate_bmc

input:
- IP address
- IPMI credentials

Try contacting the node using ipmitool with given credentials.
Return success, if succeeds to fetch power state.

*** Create the following workflow:

inputs:
- network range
- default IPMI credentials
- default driver (= pxe_ipmitool)

workflow:
- fetch IPMI addresses for existing ironic nodes
- loop over the provided network range, excluding existing nodes
-- invoke tripleo.baremetal.validate_bmc
-- if it succeeds, add a node to ironic with ipmi_address and given credentials.