Improve hiera support

This adds support for defining the following via hiera (server class params):

$mongodb::server::dbs  - a hash of databases in mongodb::db format
$mongodb::server::dbdefaults - a hash to set resource defaults for $mongodb::server::dbs (optional)
$mongodb::server::users - a hash of users in mongodb::user format
$mongodb::server::userdefaults- a hash to set resource defaults for $mongodb::server::users (optional)
$mongodb::server::replsets - a hash of replica sets in mongodb_replset provider format
$mongodb::server::replsetdefaults - a hash to set resource defaults for $mongodb::server::replsets (optional)

Added hiera hash merging support (set $mongodb::server::hieramerge to true). The default is priority based lookup.

Added a new define for creating users - mongodb::user (includes 'admin' database support)

Added support for defining multiple server installation packages (needed for MongoDB Enterprise support).