Enable Go modules as a default

[alexellis]

Description

Enable Go modules as a default

Update the templates to use GO111MODULE=on as a default

Then clarify in the docs or the README here how to turn it off, for those who need to vendor private code.

Testing

Test end to end to show that the build arg to turn off modules still works

Show output that also shows the default is working without additional settings.

[alexellis]

/add label: good first issue, help wanted

[cpanato]

i will work on that, expected to have a pr by the end of the week

[alexellis]

Thanks Carlos