Pass the user in with the psql commands
Add a default database param
Create a default_db parameter and pass to the postgresql_psql type/provider

In the case that our user is not named postgres, we have no default database to
connect to. This commit adds this so that commands can proceed using such users
as pe-postgres.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Update role.pp to use group and default_db params
Update database_grant for psql_group, default_db params