lib582: do not verify host for SFTP

This SFTP test fails with libssh back-end due to failure to verify
the peer. Disable peer verification in the test as there seems to
be the intention of the test.

Note that the libssh back-end automatically verifies the peer's
host using the default known_hosts file.

Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>