Mlab report format

This superseed #69.

In here I basically add two things:

1) Configure the policy of oonib to be the one of the first production deployment (that is only allow the http_header_field_manipulation test)

2) Configure oonibackend to write reports following the mlab convention of ` /var/spool/mlab_ooni/{YYYY}/{MM}/{DD}/{YYYYMMDD}T{HH}:{MM}:{SS}whatever.whatever`