Raise error when running ipa-adtrust-install with empty netbios--name
When running ipa-adtrust-install, a netbios-name option must be specified.
Currently if an invalid netbios name in form of empty string is specified, the
installation proceeds, but changes the invalid value to a netbios name
determined from domain name without any notification.
Fixing this so that any attempt to supply empty string as netbios name fails
with error in case of unattended installation, or to request input of valid
netbios name from command line during normal installation.
https://fedorahosted.org/freeipa/ticket/6120
Reviewed-By: Tomas Krizek <tkrizek@redhat.com>