Rework the caching mechanism

Rework the caching mechanism

Following the introduction of ./configure options and the configuration
files, this patch simplifies the caching settings:
  - the generation of the cache is done in the settings file, via
    GENERATE_CACHE (if set to "yes" it will enable the creation)
  - the cleaning of the cache is done also in the settings file, via the
    CLEAN_CACHE variable; set to empty it will disable the cache,
    otherwise it's interpreted as the number of days to keep the cache

The location of the cache has also been moved from the current dir to
under localstatedir, which is more FHS compliant (patch taken from the
debian package); this means with localstatedir=/var, the location of the
cache will be /var/cache/ganeti-instance-debootstrap.

Reviewed-by: imsnah