scheduler daemon: optimize list of local devices
scheduler daemon: optimize list of local devices

list_devices() a lot faster than get_devices() because it does not
involve reading the configuration files, and that seems to be
unacceptably slow.

Change-Id: I89b1fff22536f5e81876a3d2381aaabbc08c98e3
(cherry picked from commit cd44121e99727a0ccb61c521308e44939d3f64b0)
Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server