Fix user password validation

[alexhung]

Fixes #1031

• Fix password validation being performed for interpolated value before it is 'known'. Due to Terraform limitation, validation mechanisms don't work on interpolated value which is unknown until create/read/update/delete stage. Now the resource validates password using regular validation mechanism when it is configured as plan text, or validate just prior to creating or updating the resource. This means when using interpolated value, terraform validate or terraform plan won't report error if value is invalid.
• Make password validation logic more robust.