Add a defined type for creating database schemas

This defined type is closely modeled after
[`Postgresql::Server::Tablespace`](https://github.com/puppetlabs/puppetlabs-postgresql/blob/master/manifests/server/tablespace.pp). It uses PostgreSQL's builtin `IF NOT
EXISTS` (if available) to guarantee idempotency.

This PR addresses [MODULES-1098](https://tickets.puppetlabs.com/browse/MODULES-1098)
typo