This is needed to be able to add more context objects (see following revisions).

Added cli commands are runner, listener and api-server

These commands do what they say, ie. start a runner, listener or API server
process.

Note that processes are not daemonized and run in front.

Typically used as:

swh-scheduler --cls local --database postgresql:///?service=swh api-server --host 127.0.0.1 --port 5008

swh-scheduler --cls remote --url http://127.0.0.1:5008 runner --period 10

swh-scheduler --cls remote --url http://127.0.0.1:5008 listener

Move the scheduler verification from the main cli group definition to subcommands

since it's actually the responsibility of each subcommand to decide whether it
can run without a properly configured scheduler instance.

This is also required so the user can run:

swh-scheduler subcommand --help

even with a non-properly configured scheduler.

Also move logging configuration into the cli group function
so that logging level config can be consistently set for all swh-scheduler
commands.