ovs-pki: Use SHA-512 instead of MD5 as message digest.

This fixes numerous testsuite failures of the form "SSL_connect:
error:0D0C50A1:asn1 encoding routines:ASN1_item_verify:unknown message
digest algorithm" on systems that disable MD5 in OpenSSL.  Centos 7 is one
example.  Presumably it increase security as well for anyone who generates
certificates based on a new configuration created by the new ovs-pki.

Reported-by: Robert Strickler <anomalyst@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>