Validate vault's file parameters
    
    A user can pass file names for password, public and private key files to
    the vault plugin. The plugin attempts to read from these files. If any
    file can't be, an internal error was raised. The patch wraps all reads
    and turns any IOError and UnicodeError into a ValidationError.
    
    https://fedorahosted.org/freeipa/ticket/5155
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>