install_modules: allow to customize PUPPETFILE_DIR
install_modules: allow to customize PUPPETFILE_DIR

Allow to change PUPPETFILE_DIR but keep '/etc/puppet/modules' by
default.

Change-Id: Ie32d18d477c271b2724290edbbfd4ae7ea110480

openstack/puppet-openstack-integration,openstack/puppet-openstack-integration,openstack/puppet-openstack-integration