image-copy: add support for setting target subscription

[tamirkamara]

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

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• 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?

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[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"