[Tamir Kamara (tamirkamara)]

This adds the ability to set the desired target subscription where the new images will be created.
Resolves #203

---

This checklist is used to make sure that common guidelines for a pull request are followed.

General Guidelines

• Have you run ./scripts/ci/test_static.sh locally? (pip install pylint flake8 required)
• Have you run python scripts/ci/test_index.py -q locally?

For new extensions:

• My extension description/summary conforms to the Extension Summary Guidelines.

[Azure Builder (azuresdkci)]

If this PR is for a new extension or change to an existing extension, use the following to try out the changes in this PR:

docker run -it microsoft/azure-cli:latest
export EXT=<NAME>
pip install --upgrade --target ~/.azure/cliextensions/$EXT "git+https://github.com/tamirkamara/azure-cli-extensions.git@master#subdirectory=src/$EXT&egg=$EXT"