Improve user agent and version handling in chart-repo command

[migmartri]

• Adds an userAgent function that returns both the baked version and the application that is running it (i.e monocular), this application can be provided via a built-in flag.
• Adds a convenience version command
• Updates Makefile and circle-ci to pass the tagged release. By default unless specified, it will pass the commit sha.

$ make
docker run -it quay.io/helmpack/chart-repo:latest /chart-repo version
5d7825decbe8b1f1387b2ed916cb01013d214eac
$ make VERSION=1.0
docker run -it quay.io/helmpack/chart-repo:latest /chart-repo version
1.0

Next steps would be to update the Monocular chart to pass the new application flag.

Refs vmware-tanzu/kubeapps#767

[prydonius]

lgtm, thanks!