Add support for SaltStack Nitrogen format for per db schemas and extensions

[YetAnotherMinion]

I see warning that schemas and extensions will not work in Nitrogen version of SaltStack. I made a naive attempt to squash this error by moving the per database extensions and schemas under a context key, but that just changed the warning to context being an invalid keyword for postgres_database.present. I don't know enough about salt yet to know exactly where the postgres_database_present expects to find the context parameter.

 Warnings: 'extensions' and 'schemas' are invalid keyword arguments for
              'postgres_database.present'. If you were trying to pass additional
              data to be used in a template context, please populate 'context'
              with 'key: value' pairs. Your approach will work until Salt
              Nitrogen is out. Please update your state files.