Look for stage on both options and provider & skip execution when there are no changes

[rafaelrpinto]

This PR addresses the following issues:

Look for the stage in the options and provider objects

Currently if no stage is informed as option the deployment fails with:

Serverless Error ---------------------------------------
 
  Missing required key 'Value' in params.Tags[0]

I added a logic to look for the stage on the provider object when it is not available on the options.

Plugin throws an error message when there are no changes to deploy

Currently when there are no changes to deploy the plugin throws (described on #9 ):

Serverless: Packaging service...
Serverless: Service files not changed. Skipping deployment...
Serverless: Creating CloudFormation ChangeSet [...]...

  Serverless Error ---------------------------------------

  S3 error: Unable to get the object https://s3.amazonaws.com/...

I added a logic to check this.shouldNotDeploy variable before doing anything. This way the plugin skips the creation of the changeset when there are no changes.

[mt-micky]

@rafaelrpinto why did you close this PR? It looks like it would fix the two main issues of this plugin

[rafaelrpinto]

@mt-micky

I don't really remember why :P
Reopening as requested.

Cheers,