operator: add built-in 'Random' label templates

[fgiudici]

introduce built-in random templates:

• ${Random/UUID}
• ${Random/Hex/[0-32]}
• ${Random/Int/[MAXINT]}

examples:
${Random/UUID} --> e512d5ca-a765-42f2-82b7-264f37ffb329 ${Random/Hex/12} --> e512d5caa765
${Random/Int/1000} --> 199

Fixes #802

[anmazzotti]

Looks good with a couple of minor comments.

I also wonder if this should replace the current behavior of m-$UUID or is that already part of the PR?

[fgiudici]

Looks good with a couple of minor comments.

I also wonder if this should replace the current behavior of m-$UUID or is that already part of the PR?

Well, m-$UUID is still the default machineName, but yes, we can now generate custom machine names, e.g.:
machineName: cattle${Random/Int/5}-${Random/Hex/8}
that would generate hostnames (and machineinventory names) with names like:
cattle0-02401844
cattle3-1823bf5a