gistfile1.pp

class openstack::common::config {
  $prefix_ = 'openstack::common::config'
  $package_ensure = hiera("${prefix_}::package_ensure", 'present')
  $database       = hiera("${prefix_}::database", 'mysql')
  $database_host  = hiera("${prefix_}::database_host", undef)
}

class openstack::nova::config {
  include openstack::common::config
  $prefix_ = 'openstack::nova::config'
  $package_ensure    = hiera("${prefix_}::package_ensure", $::openstack::common::config::package_ensure)
  $database          = hiera("${prefix_}::database", $::openstack::common::config::database)
  $database_host     = hiera("${prefix_}::database_host", $::openstack::common::config::database_host)
  $database_name     = hiera("${prefix_}::database_name", 'nova')
  $database_password = hiera("${prefix_}::database_password")
}

class openstack::nova::common (
  $package_ensure = $::openstack::nova::config::package_ensure,
) inherits openstack::nova::config {

  class { '::nova':
    package_ensure => $package_ensure
  }
}

# hiera/common.yaml
# openstack::common::config::package_ensure: latest
# openstack::common::config::database_host: 10.0.0.10
# openstack::nova::config::database_host: 10.0.0.10
# openstack::nova::config::database_password: nova