Improve user input sanitation

[williamtrelawny]

At present, the only consequence of bad user input is simply either failure (if "o" or "e" are not selected in the beginning) or failure to create the proper containers after completing. The latter issue is problematic of course, but not detrimental as the user can just destroy containers and try again.

But it would improve UX if the script did basic sanity checking of user input to ensure the script doesn't complete unless:

• user successfully selected either Opensearch or Elasticsearch; AND
• user has selected valid version numbers for Graylog, MongoDB, and their indexing backend of choice.