make ca url adjustable via env variable

[thomas-mc-work]

I'd like to make the CA API URL adjustable via an environment variable. This makes it easier to cretae test scenarios by only providing the environment variable for the testing endpoint:

ACME_URL="https://acme-staging.api.letsencrypt.org" ./acme_tiny.py …

[diafygi]

DEFAULT_CA is deprecated in favor of DEFAULT_DIRECTORY_URL since the directory url is what should be used as the basis for getting the other urls. Also, if you want to use an environ variable, you can use the command line options or just call get_crt() directly with the directory_url you want to specify (that's what the test suite does).