Feature/616 len of fiware service string

### Description
Changed max length of Fiware-Service (tenant) from 20 to 50 bytes.
Also put a maximum of 13 bytes for the database name (CLI-option '-db').

I'd suggest to have this 13 in a `#define` constant.

BTW, maybe we should limit to 10 characters (easy to remember a "round member" and let some space if you need it for some reason, eg. use two characteres to separate between db and tenant "orion--service1").