Resolve variables inside string with single/double quotes.

Summary

When trying to pass an variable to a script that needs string entries that needs to be enclosured with single/double quotes it doesn't return the value.

For example

slack.sh 'Notification Message' '#channel'

Script: https://github.com/rockymadden/slack-cli

Steps to reproduce

Create a project and, in the .gitlab-ci.yml file, add the following:

Test:
  stage: notification
  variables:
    MESSAGE: '$CI_COMMIT_REF_NAME'
  script:
    - bash slack.sh chat send 'My message with variable: $MESSAGE' '#channel'

Actual behavior

Doesn't resolve the variable $MESSAGE

Expected behavior

Send the value of the CI_COMMIT_REF_NAME e.g - 'My message with variable: Master'

GitlabRunner Version

gitlab-ci-multi-runner 9.4.2 (6d06f2ec)
GitLab Enterprise Edition 9.3.1-ee a15332b