Preserve whitespaces in single-quote literal fallback

[exoego]

What did you implement:

Closes #5398

How did you implement it:

In #5594, cleanVariable does not clean whitespaces if variable is double-quote string literal.

Atter this PR, it also does not clean whitespace if variable is single-quote string literal.

How can we verify it:

1. npm install -g exoego/serverless#single-quote-fallback
2. sls deploy below and confirm it completes successfully.

service: aws-nodejs

provider:
  name: aws
  runtime: nodejs8.10

functions:
  hello:
    handler: handler.hello
    events:
      - schedule: ${env:SCHEDULE, 'rate(2 hours)'}
      - schedule: ${env:SCHEDULE, "rate(3 hours)"}

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification config / commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[pmuens]

Great! Thanks for fixing @exoego 👍

LGTM

[pmuens]

Great! Thanks for fixing @exoego 👍

LGTM