Set the correct OS disk size for Windows

[bingosummer]

Fixes #282

• Please check this box once you have run the unit tests

  pushd src/bosh_azure_cpi
    bundle install
    bundle exec rspec spec/unit/*
  popd
  

The unit test coverage is increased from 643 examples, 0 failures. 2786 / 2945 LOC (94.6%) covered. to 656 examples, 0 failures. 2798 / 2954 LOC (94.72%) covered..

Changelog

• Set the correct OS disk size for Windows
• Add a message if the version of the Windows image is not found

[cfdreddbot]

Hey bingosummer!

Thanks for submitting this pull request! I'm here to inform the recipients of the pull request that you and the commit authors have already signed the CLA.