tests: Add hostmask detection for sudo rules validating on hostmask
    
    IPA sudo tests worked under the assumption that the clients
    that are executing the sudo commands have their IPs assigned
    within 255.255.255.0 hostmask.
    
    Removes this (invalid) assumption and adds a
    dynamic detection of the hostmask of the IPA client.
    
    https://fedorahosted.org/freeipa/ticket/5501
    
    Reviewed-By: Lukas Slebodnik <lslebodn@redhat.com>
    Reviewed-By: Oleg Fayans <ofayans@redhat.com>
    Reviewed-By: Ales 'alich' Marecek <amarecek@redhat.com>