Make localstack image name configurable

[ps-jay]

This allows folks to substitute their own localstack image, or point to a local cache of Docker images - such as an Artifactory proxy/cache.

The concrete use-case for me, is the Artifactory proxy/cache:

e.g.

localstack = pytest_localstack.patch_fixture(
    services=["sts"],
    scope='module',
    autouse=True,
    region_name='ap-southeast-2',
    localstack_image='artifactory.example.org:6555/localstack/localstack',
)