Refactor role params

[Anton-Shutik]

Updated postgresql_role.

Now it supports postgres ROLE configuration params to accept string values. So you can do:

resource "postgresql_role" "role_with_configuration_parameters" {
	name = "role_with_configuration_parameters"
	statement_timeout = "2h"
	idle_in_transaction_session_timeout = "1d"
  }

Since now, 0 value is not a "placeholder", which will reset to system default value. 0 is a valid param value, which disables timeout (as for statement_timeout, idle_in_transaction_session_timeout )

Also, it is much easier to add new config params for role resource.

Fixes #353

[Anton-Shutik]

@cyrilgdn Would you mind taking a look, pls ?