Add git clone over ssh protocol

We need to have some short-hand for clonning over ssh:// protocol
this patch implements such functionality.
  Usage example:

   node {
     gitSSHCheckout {
       credentialsId = "mcp-ci-gerrit"
       branch = "mcp-0.1"
       host = "ci.mcp-ci.local"
       project = "projectcalico/calico-containers"
     }
   }

Change-Id: I5f8390cdbbd3df06cffa703190534fe10ecfd1ca
diff --git a/vars/gitSSHCheckout.txt b/vars/gitSSHCheckout.txt
new file mode 100644
index 0000000..087d018
--- /dev/null
+++ b/vars/gitSSHCheckout.txt
@@ -0,0 +1,27 @@
+// Default parameters
+
+  target_dir = "./"
+  port = "29418"
+
+// Usage example
+node {
+  gitSSHCheckout {
+    credentialsId = "mcp-ci-gerrit"
+    branch = "mcp-0.1"
+    host = "ci.mcp-ci.local"
+    project = "projectcalico/calico-containers"
+  }
+}
+
+// or
+
+node {
+  gitSSHCheckout {
+    credentialsId = "mcp-ci-gerrit"
+    branch = "mcp-0.1"
+    host = "ci.mcp-ci.local"
+    project = "projectcalico/calico-containers"
+    target_dir = "some_directory"
+  }
+}
+