dsc port_base and port_step need updates

[leftwo]

port_base must be know by dsc if it is trying to start an existing region.
port_step is hard coded to be 10 and can't be configured from the command line, but
we do include it as a field in Regions.

Both of these could possibly live in a dsc "info" file (much like Crucible's region.json)
This could also enable recording other information, like number of expected regions, encryption, and
other details that would make starting dsc simpler and require the caller to remember less.

It would also enable tests to be more generic and just take whatever dsc regions were created
by a previous run and start up downstairs on them, with the knowledge that dsc could figure out
what exactly it needs to start.

port_step should also be configurable from the command line

[leftwo]

If we allow a custom port_step, we need to update all the tests in ./tools, so perhaps those tests
should transition to using dsc directly were possible.