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

ci-scripts/cls_oaicitest.py

@@ -3179,7 +3179,7 @@ class OaiCiTest():
 				result = re.search('warning: discard PDU, sn out of window', str(line))
 				if result is not None:
 					nbPduDiscard += 1
-				result = re.search('--nfapi 5 --node-number 2 --sa', str(line))
+				result = re.search('--nfapi STANDALONE_PNF --node-number 2 --sa', str(line))
 				if result is not None:
 					frequency_found = True
 			result = re.search('Exiting OAI softmodem', str(line))