Use string values for nFAPI parameter (--nfapi)

- previously, used magic numbers to designate nFAPI mode
- now, use string values (MONOLITHIC, PNF, VNF, ...)
- Remove printing of the nfapi mode to the console, as it's value is clear from the command-line argument
- Update usages of the --nfapi command-line parameter in scripts and documentation to reflect the change to string values