cloud_tests: add impish release definition

[paride]

Proposed Commit Message

cloud_tests: add impish release definition

Additional Context

Spotted in our integration testing.

Test Steps

On main:

$ python3 -m tests.cloud_tests run --os-name=impish --platform=nocloud-kvm --preserve-data --data-dir=results --verbose                                2021-06-18T16:06:28 CEST
usage: cloud_tests run [-h] [-p PLATFORM] [-n NAME] [-t FILE] [--feature-override FEATURE_OVERRIDE] [-v] [-q] [-r FILE] [-d DIR] [--preserve-instance] [--preserve-data] [--deb FILE] [--rpm FILE] [--script DATA]
                       [--repo NAME] [--ppa NAME] [-u] [--upgrade-full]
cloud_tests run: error: argument -n/--os-name: invalid choice: 'impish' (choose from 'artful', 'bionic', 'centos66', 'centos70', 'cosmic', 'disco', 'eoan', 'focal', 'groovy', 'hirsute', 'jessie', 'stretch', 'trusty', 'xenial')

With this PR:

$ python3 -m tests.cloud_tests run --os-name=impish --platform=nocloud-kvm --preserve-data --data-dir=results --verbose              2021-06-18T16:07:07 CEST
2021-06-18 14:07:09,141 - /home/paride/ubuntu/git/cloud-init/tests/cloud_tests/__main__.py:main:54 [DEBUG]: running with args: [...]

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

[TheRealFalcon]

Good catch, thanks!