Also remove system interface check from ensureSupervisorNetwork.
Previously `ensure` was a Bluebird promise which wasn't awaited in
its composition step. This has been here for some time and may contribute
to issues with duplicate networks. The conversion to native Promises
allows `ensure` to be awaited, hopefully reducing instances of duplicate
networks.
Removing the system interface check for /sys/class/net/supervisor0
because it's superfluous given that the Engine creates the interface
with NetworkManager. It also makes testing a lot more difficult to set up
as /sys/class/net isn't a directory that can be written to for emulating
system interface creation / removal.
Relates-to: https://github.com/balena-os/balena-supervisor/issues/1110
Change-type: minor
Signed-off-by: Christina Ying Wang <christina@balena.io>
