By popular request (i.e. Bill's), the 'skycal' stage has been added.

The mysql script of this directory is supposed to be run on each node
hosting a nebulous database. When executed on the DisksMonitoring
database, it adds a new value to the 'stage' enum field of all tables
neb_<disk_id>_[00-ff]: 'skycal'.

The files built at the 'skycal' stage match the pattern '\.sky\.'.

