More Gitlab CI improvements

[dhollinger]

Added following improvements:

• .gitlab-ci.yml documentation added to the README.md
• Added code to .gitlab-ci.yml.erb to allow for users to override the defaults completely if they so choose.

[DavidS]

@cdenneen I can't add you as a reviewer, but can you give 👍 here, if this change is ok now?

[cdenneen]

@dhollinger anyway to make that matrix more of a "matrix" (array) rather than all the duplication of lines?

[cdenneen]

global variables look good 👍

[dhollinger]

@cdenneen I'm thinking of how to do that still before I add anything. It would take some work as Gitlab CI doesn't support matrices the way Travis does. Gitlab CI operates in Pipelines (connected jobs) rather than a bunch of one-off "build" jobs (Just comparing to my experience with both).

[cdenneen]

yeah so you'd still have something like

stages:
  - validate
  - metadata
  - unit
  - acceptance

ruby_versions:
  2.1.9:
  puppetversion: 4.10.11
  2.4.3:
  puppetversion: 5.5.1

and then some way of doing

for each stage
for each ruby version
for each puppet version

so the jobs come out as

validate puppet 4
validate puppet 5
metadata puppet 4
metadata puppet 5
unit puppet 4
unit puppet 5
acceptance puppet 4
acceptance puppet 5