Use golang 1.12.12

Use use base official golang image in CircleCI, as it gets refreshed
more quickly compared to circleci/golang, and we don't benefit from
circleci image customizations much.

To get fix for https://github.com/golang/go/issues/34540 .

Fix two broken links in the Documentation & Guides section, namely Advanced Job Scheduling on Nomad with Affinities and Increasing Nomad Fault Tolerance with Spread

The default ulimit for open files on macOS is really small (256),
which leads to "too many open files" errors during plugin launches if
you're running unit tests in `drivers/docker`. Recommend setting
`ulimit -n 1024`.

It's the latest 1.11 point release as of now

In commit 758e1cb7 the `strings.Builder` API was used; this isn't available until Go 1.10+

`sudo` is required to enable other drivers like rkt and java while the
`-dev` flag enables the agent to run in a standalone mode.

Change Ubuntu version used in Vagrant for development purpose.