Rework package installation during post-boot configuration

Rework package installation during post-boot configuration

* New script install_packages.sh is the single entry-point for
  installing packages outside kickstart.

* New configuration variable CONFIG_EXTRA_PACKAGES has space-separated
  list of extra packages to install.  This can help if dependencies
  are broken.

* Rename install_gpfs_nas.sh to just install_nas.sh, now just installs
  packages (e.g. samba, ctdb, rssh) to support protocols.

* Rename install_gpfs.sh to install_clusterfs_gpfs.sh to provide a
  hook to support installation of alternate cluster filesystem
  packages.

Signed-off-by: Martin Schwenke <martin@meltin.net>