String Interpolation with a custom command

@cheynearista

How do I implement a custom command where I pass a string to the GetCmd() function?

In my case below I want to build a command that returns only a single interface - testing with Ethernet1

```
type ShowInterfaceDescription struct {
InterfaceDescriptions map[string]InterfaceInfo `json:"interfaceDescriptions"`
}

type InterfaceInfo struct {
InterfaceStatus string `json:"interfaceStatus"`
Description string `json:"description"`
LineProtocolStatus string `json:"lineProtocolStatus"`
}

func (s *ShowInterfaceDescription) GetCmd() string {
i := "Ethernet1"
return fmt.Sprintf("show interfaces %v description", i)
}
```

Start with the GetCmd() method and the custom command example in the issue. Determine how a caller should provide "Ethernet1" when constructing the command, then verify that the resulting command is "show interfaces Ethernet1 description"; the issue names no files or tests to guide further work.

Written by the indexing model from the issue text.