mender-convert: Production polling intervals

mender-convert uses the same polling intervals as the demo settings of meta-mender.
Over time this can lead to stressing the server.

Acceptance criteria

• demo (non-production) mode is introduced with the `--demo` parameter (off by default)
• when demo mode is used, mender-convert uses the same polling intervals as when meta-mender's demo layer is added (NB! make sure to see if it affects cause for this related PR - may need longer retries than meta-mender's demo layer)
• when demo mode is not used, mender-convert uses the same polling intervals as meta-mender
• Update Mender Hub article to use the new demo parameter