ovs-pki: Use SHA-512 instead of SHA-1 as message digest.

ovs-pki: Use SHA-512 instead of SHA-1 as message digest.

The upcoming OpenSSL 1.1.0 release disables use of SHA-1, which breaks the
OVS unit tests, which use SHA-1.  We last tried to switch to SHA-512 in
2014 with commit 9ff33ca75e9fcc ("ovs-pki: Use SHA-512 instead of MD5 as
message digest."), but we had to downgrade to SHA-1 in commit 4a1f9610682d
("ovs-pki: Use SHA-1 instead of SHA-512 as message digest.") because
XenServer did not support SHA-512.  It has been a few years, so let's try
again.

CC: 828478@bugs.debian.org
Reported-at: https://bugs.debian.org/828478
Reported-by: Kurt Roeckx <kurt@roeckx.be>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Ryan Moats <rmoats@us.ibm.com>