Fix token secret length RFC compliance

RFC 4226 states the following in section 4:
   R6 - The algorithm MUST use a strong shared secret.  The length of
   the shared secret MUST be at least 128 bits.  This document
   RECOMMENDs a shared secret length of 160 bits.

Reviewed-By: Jan Cholasta <jcholast@redhat.com>

ipalib/plugins/otptoken.py

@@ -59,7 +59,7 @@ TOKEN_TYPES = {
 }
 
 # NOTE: For maximum compatibility, KEY_LENGTH % 5 == 0
-KEY_LENGTH = 10
+KEY_LENGTH = 20
 
 class OTPTokenKey(Bytes):
     """A binary password type specified in base32."""