Assigns default nameserver in sandbox service.

[jglogan]

• Closes Network issue occurs when container build process installs pip packages #148.
• Storing the default nameserver in the bundle config means that DNS won't work if the container stops and then restarts later when the subnet address has changed.

[crosbymichael]

This works for our current implementation but adding this note for future us; assuming the gateway is the name server could cause a bug later on. We should probably look at making this more explicit within the network services to be more robust.

[jglogan]

pol> assuming the gateway is the name server could cause a bug later on

Yep, this definitely assumes a vmnet network. For our future us, one possibility is that the attachment descriptor returned by the network helper could include a nameservers hint, if the network implementation can suggest one. We can always override the default policy by specifying nameservers explicitly when creating the container.