test(rspconfig): cover one network value per BMC

Extract per_bmc_argument and the set of settings that take a list from
ipmi.pm, and drive the real routine, because the plugin needs daemon
dependencies that the test host does not have.

Cover a single value serving every BMC, a list read in session order, a
list too short for the BMC being configured, empty entries, and the
defensive inputs. Assert that only ip, netmask and gateway read a list,
that the caller reports a short list rather than sending it, that the
session keeps the value of its own BMC for the follow-up, and that the
thermal profile keeps its own argument.