tls: use SHA1 for sessionIdContext

FIPS 140-2 disallows use of MD5, which is used to derive the
default sessionIdContext for tls.createServer().

PR-URL: https://github.com/nodejs/node/pull/3866


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: James M Snell <jasnell@gmail.com>