Use setProperty function in fan set speed

When a fan's target speed is set, using the setProperty function
allows for better handling of dbus failures setting the fan target
property. If a failure occurs setting this property, a journal entry is
created and a DBusPropertyError exception is thrown to allow the fan
control application to exit and restart quickly in its allowed attempts
configured in systemd.

Tested:
    Fan control application logs an error to the journal and then
terminates when a dbus error occurs setting a fan target speed.

Change-Id: Ibd4bd8b18b6010727831d97e32c14fd6c681e170
Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
1 file changed