Allow multiple validators

Switch User/Password/Role validators to IList instead of a single instance, this will allow us to split apart functionality i.e. (UniqueEmail, UniquePhoneNumber) instead of requiring one mega validator which should help with some of the validation extensibility issues

cc @divega @suhasj  
We aren't trying to block the scenario where they want to have no validation, if a developer wishes to have no manager level validation, they should have a way to do it.