use directory structure matching the project group names

[Fathi Boudra]

On offspring.linaro.org, we're using the following structure with the project name:
http://snapshots.linaro.org/build/lt-origen-oneiric/
http://snapshots.linaro.org/oneiric/lt-origen-oneiric/

we would like to use directory structure matching the project group name:
http://snapshots.linaro.org/build/${project_group_name}/${project_name}
http://snapshots.linaro.org/${project_group_name}/${project_name}

It will be nice to remove the need to request syncs from IS.