control: Support `member` signal triggers

`member` signals are signals that a dbus object could send out on dbus
that acts as a notification that does not contain any other data. This
adds support to fan control events that can be triggered by this type of
signal on dbus to essentially run a set of actions. The configuration of
this trigger is contained within the JSON configuration of the trigger
on the event and not against the event's group(s).

i.e.)
"triggers": [
  {
    "class": "signal",
    "signal": "member",
    "member":
      {
        "name": "Pressed",
        "path": "/xyz/openbmc_project/Chassis/Buttons/Power0",
        "interface": "xyz.openbmc_project.Chassis.Buttons.Power"
      }
  }
]

Change-Id: Ie7924174333c604fb54c8f18560a890bf6a7c489
Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
3 files changed