Add Gitlab CI jobs

[jeffmccune]

Without this patch the control repository does not contain any scripts to
execute CI jobs for Puppet code deployment. This patch addresses the problem
by adding a Gitlab CI job configuration with three jobs defined across two
stages. The test stage executes first with a Syntax and a Lint job executing
in parallel. If an environment branch has been pushed, one matching
developemnt, testing, production or starting with the prefix
playground, the deploy stage is executed.

The deploy stage requires a Gitlab secret environment variable, PUPPET_TOKEN.
This environment variable is the puppet access token configured with Code
Manager.

The test stage runs the following syntax checks:

• yaml files (*.yml, *.yaml)
• json files (*.json)
• bash scripts (*.sh)
• ruby scripts (*.rb)
• puppet manifests (*.pp)
• erb files (*.erb)

The behavior of the checks are to check only files modified relative to
a base branch, defaulting to production. This avoids running syntax
checks against files which have not been modified by the merge request.
The check uses git diff --name-status to identify changed files across
multiple commits in a topic branch.

The lint checks rely on bundler and the Gemfile to install numerous lint
checks.

Gem libraries are installed in a per-job location in $HOME. This is an
intentional compromise between installing into a system GEM_PATH
element, which would create library conflicts with other jobs, and the
job workspace, which would cause gem libraries to be installed from the
network on each job invocation.

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