Pass DEBUILDER_BUILDDIR to pbuilder via --buildplace
Remove separate rebuildd source command

Build command specifies the full path to .dsc file, no need to pull it
seprately.
Configuration now done automatically on startup, again

Build environment initialization is now separated from the configuration
process, and needs to be done by sysadmin in advance.

Debuilder searches for build environments and configures its
distributions and architectures based on found tgzs.
Revise README
Use common aptcache dir for all builds
Add config variable for rebuildd threads
Add config variable for dpkg-buildpackage options