Add a script to generate, store and retrieve a valid EC2 ssh key

[danjoneslf]

• [x] Add create_keypair.sh script
• [x] Add retrieve_keypair.sh script
• [ ] ~Run in local exec~
  (I decided not to do this because I couldn't get the depends_on to work properly so I've just put it in the pipeline)
• [x] Store keypair in secretsmanager as version of existing secret (Secrets manager does not handle destroy/apply well)
• [x] Test apply with the generated key
• [x] Test destroy with the retrieved key
• [x] Add an issue to manage the secret in the exec_roles terraform