Adding bootstrapping config, and factoring out projects in a common file

This pull request adds a number of changes to the community-builds configuration.

1) the generation of the scala artifacts is separated from the remaining projects, which are moved to a new file "common-2.11.x.conf". This file can be included from different configurations, making it simpler to test the same set of projects using different options (for instance -optimize, or GenBCode, etc).

2) a new configuration "community-bootstrap" is added, which includes the common projects as described above. The bootstrap configuration is able to compile the full set of compiler and modules with itself, and also checks for stability. For complete information, please refer to the content of community-bootstrap-2.11.x.dbuild

3) in order to make it easier to change the dbuild version in the various configurations, and to tune other parameters, a script is added for each configuration, invoking dbuild-runner as needed. The dbuild version for each configuration can be therefore changed via Git, and the Jenkins job will only call this new script.

4) other minor changes and improvements to dbuild-runner.

/CC @retronym, @gkossakowski, @adriaanm,