added new features

- expanding `environment` vars in cmd section
- support for connecting to one proxy/bastion host
- better notification text layout
- better error message on private key failing to open

pkg/backy/templates/success.txt

@@ -0,0 +1,7 @@
+Command list {{ .listName }} was completed successfully. 
+
+
+The following commands ran:
+{{- range .CmdsRan}}
+    - {{. -}}
+{{end}}