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

Commit 9ff33ca7 (ovs-pki: Use SHA-512 instead of MD5 as message
digest.) changes the message digest algorithm to SHA-512.  This
seems to break the unit tests on some xenserver 5.6/6.0 builds
causing the error: "SSL_connect: error:0D0C50A1:asn1 encoding
routines:ASN1_item_verify:unknown message digest algorithm".

As a solution, this commit changes the message digest algorithm
to SHA-1 which works for both the above xenserver builds and
centos 7.

VMware-BZ: #1319116

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>