Restructured credential generation with new sourceable secrets file

[dsingh07]

Credential generation now does the following:

• A secrets file is now generated with all the randomly generated passwords as well as the initial admin credentials
• This file is now sourced everytime credentials.generate.sh is called so that all the environment variables are in the shell
• An example file is provided which is used to create the secrets file
• The secrets file should not be uploaded to a remote repo hence it is in the gitignore
• Secrets file should be deleted and recreated for each new ADOP instance, hence if the secrets file is changed, docker compose needs to be re-run
• Additional strength checks for the user password

[nickdgriffin]

LGTM.