trusts: Detect missing Samba instance
In the event of invocation of trust related commands, IPA server needs to
contact local Samba instance. This is not possible on servers that
merely act as AD trust agents, since they do not have Samba instance
running.
Properly detect the absence of the Samba instance and output
user-friendly
message which includes list of servers that are capable of running
the command, if such exist.
List of commands affected:
* ipa trust-add
* ipa trust-fetch-domains
* all of the trustdomain commands available via CLI
https://fedorahosted.org/freeipa/ticket/5165
Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>