Make `wr mem` or equivalent optional

[lykinsbd]

Today, when Stockpiler backs up a network device, it saves the running configuration to startup (or whatever is the equivalent on a given platform).

We need to make this behavior optional.

[lykinsbd]

In https://github.com/lykinsbd/stockpiler/blob/master/stockpiler/tasks/stockpile/stockpile_cisco.py#L26 we would need to add an argument for "save_config" or similar, defaulting to True.

Then based on if that bool, we would make this chunk conditional: https://github.com/lykinsbd/stockpiler/blob/master/stockpiler/tasks/stockpile/stockpile_cisco.py#L73