commands: remove choice checking for --output-format in cmd_ phase
The problem was introduced with my "commands:cmd_session: don't
get_exporter_names() when not needed" 84e3b2c96566df7322a2dd661c260b60604b55bc.
For each subcommand that we have in plainbox we have split them to (at least)
two files. cmd_* and inv_*. The former having the declaration of arg and its
options and the latter having the implementation. This ensures that every
module that invocation needs is only loaded when we invoke the command.
With my 84e3b2 patch I removed checking of all available exporters in the cmd_
phase. This led to a plainbox starting up 4x faster. I left '?' as the only
possible value of --output-format, and when invoked with that arg, it would
actually pull all exporters from providers and list them, as it would run
through the inv_ phase.
unit:job: Add the siblings creation feature to JobDefinition
Using this field, creating slightly modified versions of a job does not require
duplicating it then changing just a field or two.
Typical use of this feature are job definitions for test we have to run before
and after suspend or even test that we usually replicate for multiple ports
when there's no resource to do the enumeration.
The syntax of this field must be a JSON array of dictionaries where the
keys/value pairs override or add fields to the base job.
