specify config containing keys via flag to services

[0pcom]

with reference to the README documentation for skywire-deployment

https://github.com/skycoin/skywire-deployment/

most of the services are started with the flag --sk $(tail -n1 /etc/some.config)

To avoid needing to use external utilities when invoking the service, we should allow the conf file containing keys (which is not actually json) to be specified directly via flag, and the secret key should be parsed from the last line of that file.

NOTE: systemd services for the skywire services will be included in the package with the aforementioned tail -n1 method for now ; which will basically require bash at that point. In order to eliminate those spurious deps in the services, we should implement the flag described in this ticket.