Support custom renewal statements in Postgres

[ConstantineXVI]

This is to support a quirk of Redshift that prevents the standard ALTER ROLE from working correctly.

(Related: I want to add the option of adding caps to the generated passwords to meet Redshift's password requirements; but not immediately obvious where to make that configurable)

[calvn]

I think you can do something similar to https://github.com/hashicorp/vault/blob/master/plugins/database/mysql/mysql.go#L165 and have a const defaultRenewUser instead having the two methods customRenewUser and defaultRenewUser to reduce code duplication.

[ConstantineXVI]

Valid. Rewritten to eliminate the split.

[calvn]

LGTM!

[ConstantineXVI]

I think I inherited those Travis failures from master when I forked. At least I'm hoping I didn't somehow break http from my changes.

[briankassouf]

Looks good to me! Thanks for the contribution!

[briankassouf]

@ConstantineXVI As for the password requirements. You could so something similar to how mysql has multiple implementations for different versions of mysql each with their own username requirements. See https://github.com/hashicorp/vault/blob/master/helper/builtinplugins/builtin.go#L17 for an example