Stage is added twice and erroneous

[StephanPraetsch]

Hi there

if I define the stage with a variable the stage is added twice and the second add is erroneous because the variable is not mapped
myLambda-myStage-${self:custom.stage}

reproduce

serverless create --template aws-java-maven --path myLambda

in the created directory:
change serverless.yml to

service: myLambda
custom:
  stage: myStage
provider:
  name: aws
  runtime: java8
  stage: ${self:custom.stage}
package:
  artifact: target/hello-dev.jar
functions:
  hello:
    handler: com.serverless.Handler
plugins:
  - serverless-aws-alias

then run commands

mvn clean package
npm install --save serverless-aws-alias
serverless deploy -v

The output of the deploy step is

Serverless: Packaging service...
Serverless: Preparing alias ...
Serverless: Processing custom resources
Serverless: Removing resources: 
Serverless: Processing functions
Serverless: Creating Stack...
Serverless: Checking Stack create progress...
CloudFormation - CREATE_IN_PROGRESS - AWS::CloudFormation::Stack - myLambda-myStage
CloudFormation - CREATE_IN_PROGRESS - AWS::S3::Bucket - ServerlessDeploymentBucket
CloudFormation - CREATE_IN_PROGRESS - AWS::S3::Bucket - ServerlessDeploymentBucket
CloudFormation - CREATE_COMPLETE - AWS::S3::Bucket - ServerlessDeploymentBucket
CloudFormation - CREATE_COMPLETE - AWS::CloudFormation::Stack - myLambda-myStage
Serverless: Stack create finished...
 
  Serverless Error ---------------------------------------
 
     The stack alias name "myLambda-myStage-${self:custom.stage}"
     is not valid. A service name should only contain alphanumeric
     (case sensitive) and hyphens. It should start with an
     alphabetic character and shouldn't exceed 128 characters.
 
  Get Support --------------------------------------------
     Docs:          docs.serverless.com
     Bugs:          github.com/serverless/serverless/issues
     Forums:        forum.serverless.com
     Chat:          gitter.im/serverless/serverless
 
  Your Environment Information -----------------------------
     OS:                 linux
     Node Version:       6.9.1
     Serverless Version: 1.13.2

Best regards

[HyperBrain]

Could you add the callstack (printed when SLS_DEBUG=* is set)?

The expectation here would be that the alias plugin creates a second stack (myLambda-myStage-myStage) that acts as the master alias stack.

It might be that the retrieval of the stage name does not cover the custom definition of a stage via variables. I will try to reproduce it, and check how and if the variable substitution is done in this case.

[HyperBrain]

The stage and alias names should now be extracted correctly in case the stage is declared as variable.