integration_tests: add SSH key selection settings

[OddBloke]

integration_tests: add SSH key selection settings

This introduces PUBLIC_SSH_KEY, to configure what public SSH key should
be passed to (and used to access) systems under test, and KEYPAIR_NAME,
to configure the name used in clouds for that SSH key (or the default
SSH key, in PUBLIC_SSH_KEY's absence).

Test Steps

To test both parts together, register a non-default SSH key in EC2 with a key name other than your username, then run:

CLOUD_INIT_KEYPAIR_NAME=<EC2 key name> CLOUD_INIT_PUBLIC_SSH_KEY=/path/to/your/created/public/key pytest tests/integration_tests/modules/test_ca_certs.py

Verify that the settings have been correctly picked up (by examining the logged output), and then confirm that the test passes.

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