install: server-install: abort installation on permission issue
    
    Abort installation if we do not have permissions to open the install
    log for writing.
    
    We use an environment variable to get the location to log the install
    output to, since at the time the logging is initialized, the arguments
    to ipsilon-server-install have not yet been parsed.
    
    Fixes: #207
    Signed-off-by: Patrick Uiterwijk <puiterwijk@redhat.com>
    Reviewed-by: Pierre-Yves Chibon <pingou@pingoured.fr>