[cli] Resolve ENV placeholders in secrets.yaml with a bash-like syntax

[eolivelli]

Summary:

• now in your secrets.yaml file you can use the bash syntax for ENV variables ${VARIABLE:-defaultValue}
• reworked the sample secrets.yaml file and instances/astra.yaml to use ENV variables
• reworked all the sample applications to instruct the user to export ENV variables with the API keys, tokens, database names...
• reworked some cases in which we used "is null" vs "is empty", now with the default secrets many variables are defined but empty, especially the Astra secureBundle, database, token, username, password...
• removed some useless example applications

This patch contains unrelated changes to:

• debug statements (less verbosity)
• classpath of the Cassandra Drivers (the WebCrawler application crashed due to a problem on the class path)

[nicoloboschi]

space

[nicoloboschi]

Suggested change

	export ASTRA_TOKEN=...
	export ASTRA_TOKEN=AstraCS:xxxx

[eolivelli]

I don't think it is worth, the user has to copy/paste from the Astra UI, and we are now using the same terms: token, secret, clientId. Previously we called them username/password with a custom mapping to clientId and secret

[nicoloboschi]

shall we default to openai here instead of azure ?

[eolivelli]

yes, I have updated it

[eolivelli]

actully I have removed the whole application, we already have other applications, like the WebCrawler, that do the same things.

It is confusing to have too many examples that perform almost that same things

[nicoloboschi]

Suggested change

	export ASTRA_TOKEN=...
	export ASTRA_TOKEN=AstraCS:xxx