Ensure correct order in postgresql::user
The password update exec can be applied first, in which case
Puppet skips it since the user does not exist, and the user is
created with the wrong credentials. The next puppet run would
fix this, but anything depending on it will still break.
Since the pass_set exec intentionally avoids depending on anything,
use a notify instead of a require to ensure ordering. This might
result in the command running twice, but there is no harm in that.