ipa-replica-conncheck: handle ssh not installed
    
    When ipa-replica-conncheck is run but ssh is not installed, the tool exits
    with a stack trace. Properly handle the error by raising an Exception in the
    SshExec constructor, and catch the exception in order to ignore the error and
    skip ssh test.
    
    The tool will exit with the following output:
    [...]
    Check RPC connection to remote master
    trying https://master.domain.com/ipa/session/json
    Forwarding 'schema' to json server 'https://master.domain.com/ipa/session/json'
    Retrying using SSH...
    WARNING: ssh not installed, skipping ssh test
    
    https://pagure.io/freeipa/issue/6935
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>