ovs-monitor-ipsec: Fix _nss_clear_database() parse error.

ovs-monitor-ipsec: Fix _nss_clear_database() parse error.

_nss_clear_database() runs `certutil` in order to get a list
of certificates currently loaded in NSS. This fails with error:

"ovs-monitor-ipsec | ERR | Failed to clear NSS database.
startswith first arg must be bytes or a tuple of bytes, not str"

Modify subprocess.Popen() to write in 'text' mode so that
'startwith' can correctly parse output.

Signed-off-by: Mark Gray <mark.d.gray@redhat.com>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>