ssh: fix compatibility with recent OpenSSH

Make x/crypto/ssh tests compatible with recent OpenSSH versions.
This means not using rsa keys shorter than 1024 bits any more, and
explicitly enabling all key types in the OpenSSH config, since some
are now disabled by default.

Tested against OpenSSH_7.2p2 and now passes.

Change-Id: I607bf15f063d4833b0876393d6845a4f507f48af
Reviewed-on: https://go-review.googlesource.com/22840
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ssh/test/test_unix_test.go

@@ -46,6 +46,7 @@ TrustedUserCAKeys {{.Dir}}/id_ecdsa.pub
 IgnoreRhosts yes
 RhostsRSAAuthentication no
 HostbasedAuthentication no
+PubkeyAcceptedKeyTypes=*
 `
 
 var configTmpl = template.Must(template.New("").Parse(sshd_config))