overlay coreos/config: Start installing default openssh configs

The configs provided by the openssh and Gentoo projects usually keep
everything commented out, meaning that the default values will be
used. On top of that, they will also include snippets in the snippet
directory. As such, start installing the default configs. We only mask
a snippet that defines sftp subsystem, because we provide our own
config snippet from coreos-base/misc-files that defines it.

sdk_container/src/third_party/coreos-overlay/coreos/config/env/net-misc/openssh

@@ -1,12 +1,9 @@
-# We install these with our chromeos-base package.
+# Do not install the setuid file in production images.
+#
+# Do not install the config snippet that defines a subsystem. We have
+# our own definition in coreos-init.
 if [[ $(cros_target) != "cros_host" ]] ; then
-	openssh_mask="
-		/etc/ssh/ssh_config
-		/etc/ssh/sshd_config
-		/etc/ssh/ssh_config.d
-		/etc/ssh/sshd_config.d
-		/usr/lib*/misc/ssh-keysign
-	"
+	openssh_mask="/usr/lib*/misc/ssh-keysign /etc/ssh/sshd_config.d/*gentoo-subsystem.conf"
 	PKG_INSTALL_MASK+=" ${openssh_mask}"
 	INSTALL_MASK+=" ${openssh_mask}"
 	unset openssh_mask