Fix issue with depulicate ssh keys after save

The CloudConfig.Save() did not load the metadata resulting in the diff
function always thinking the SSH keys were new.  This would cause them
to be saved over and over again just getting bigger and bigger.

config/config.go

@@ -163,6 +163,9 @@ func (c *CloudConfig) Save() error {
 	if err != nil {
 		return err
 	}
+
+	exCfg = mergeMetadata(exCfg, readMetadata())
+
 	exData := map[interface{}]interface{}{}
 	if err := util.Convert(exCfg, &exData); err != nil {
 		return err

tests/integration/rostest/test_08_ssh_key_merge.py

@@ -0,0 +1,43 @@
+import pytest
+import rostest.util as u
+from rostest.util import SSH
+
+
+@pytest.fixture(scope="module")
+def qemu(request):
+    q = u.run_qemu(request)
+    u.flush_out(q.stdout)
+    return q
+
+
+def test_ssh_key_merging(qemu):
+    SSH(qemu).check_call('bash', '-c', '''cat > test-merge << "SCRIPT"
+set -x -e
+rm /var/lib/rancher/conf/cloud-config.yml
+
+EXISTING=$(ros config get ssh_authorized_keys | head -1)
+cat > /var/lib/rancher/conf/metadata << EOF
+SSHPublicKeys:
+  "0": zero
+  "1": one
+  "2": two
+EOF
+ros config set hostname one
+ros config set hostname two
+ros config set hostname three
+
+cat > expected << EOF
+$EXISTING
+- zero
+- one
+- two
+
+EOF
+
+ros config get ssh_authorized_keys > got
+
+diff got expected
+
+SCRIPT
+sudo bash test-merge
+    '''.strip())