Redefine bbp repo defaults and location

Use a default settings file (bbp.epfl.ch.conf) and remove the definition
of CVM_HTTP_PROXY.

Signed-off-by: Carlos Aguado Sanchez <carlos.aguado@epfl.ch>
Sanitize input parameters

Signed-off-by: Carlos Aguado Sanchez <carlos.aguado@epfl.ch>