Sanitize NSSAI configuration in the gNB conf file

solves #548 (closed)

• SD is optional: when value is 0xffffff or not present in config (previously: needed to be set to 0)
• Checks SSTs in range [0,255]
• Checks SDs in range [0,0xffffff]
• Correct typo SST defines
• all conf files have been modified to only include a single, standardized SST value 1 (eMBB) without an SD

Regarding the last point: I modified all config files to only have an SST without an SD (standardized eMBB). Currently, the OAI 5GCN ignores the SD if the SST is in [0,127]. However, as described in #548 (closed), this is not standard-compliant. If they implement the right behavior, and taking into consideration that the SD is pre-configured in all CN docker-compose files, this will likely lead to NGAP setup failures everywhere. Thus, we either

1. go through the pain that users have to upgrade or
2. we revert the NSSAI values in all configs, but this feels wrong given that the RAN does not yet handle multiple slices and a standardized eMBB would semantically make much more sense.