Add --use-external-mip-script flag

- add a command line flag to gnt-cluster init and modify to change the
  value of the cluster parameter use_external_mip_script;
- add two constants representing the paths of the default script and of
  the external script;
- add documentation for the new option in the gnt-cluster man page;
- add use_external_mip_script to gnt-cluster info output.

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>