REF sandbox.py -> run_job.py
Refactor configuration class

Instead of InifileConfiguration class, simply parse the inifile once
with a schema and pass around resulting dict. This was done to make
it easy to serialize the configuration and put it in HDIST_CONFIG.
REF major refactor to make Builder use new run_job

The reason this got a bit involved is that in order to localize
environment setup to run_job only, Builder now inserts new commands
for unpacking 'sources' and writing 'files'.