Allow variable resolving

[AhmedNourJamalElDin]

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Fixes issue #[229]

Changes:

Based on below this (section)[https://serverless.com/framework/docs/providers/aws/guide/plugins#serverless-instance], which is:

Note: Variable references in the serverless instance are not resolved before a Plugin's constructor is called, so if you need these, make sure to wait to access those from your hooks.

So I moved the calls to this.config to the function they need them

Demo Link: No demo

Screenshots for the change: check the changes tab in the pull request