Add feature to let user decide whether or not to continue using ngcacher proxy after snaps-boot

[spisarski]

Currently the apt.conf on all of the nodes deployed by snaps-boot will have the proxy settings for ngcacher on the build server. This should be fine functionally; however, some users may not want to have their apt traffic run through another proxy server or any one at all.

We should have some sort of post installation script that can optionally change that setting on the nodes.