Track app version in Chart.yaml

This MR adds the appVersion value to the Chart.yaml file. Part of gitlab-org/gitlab-runner#3782 (closed).

Configuring appVersion will help users discovering proper versions of the chart. Since we can't easily re-use Runner version for Runner's Helm Chart version (please see discussion in !72 (merged) for context), for now we will stick with current version numbering. So what users see now, when they try to find out which versions of Runner's chart are available?

$ helm search gitlab-runner -l
NAME                    CHART VERSION   APP VERSION     DESCRIPTION  
local/gitlab-runner     0.1.44                          GitLab Runner
local/gitlab-runner     0.1.43                          GitLab Runner

We see that there are two versions available - 0.1.43 and 0.1.44. But which versions of GitLab Runner they contain? There is no way to discover this other than tracking the sources of Helm Chart or installing one of them.

When we will start using appVersion, the output will be changed significantly:

$ helm search gitlab-runner -l
NAME                    CHART VERSION   APP VERSION     DESCRIPTION  
local/gitlab-runner     0.1.44                          GitLab Runner
local/gitlab-runner     0.1.43          11.6.0          GitLab Runner

With this, finding out which version of the chart contains requested version of GitLab Runner is super easy.

Additionally, this MR makes the appVersion value the single source of truth about which version of the used version of GitLab Runner. The .Chart.AppVersion value is used to create the default value of image used by this chart. So instead of updating Chart.yaml file and values.yaml file (the image: value), the only thing that needs to be updated is Chart.yaml.

With this change update of used Runner version (e.g. similar to !69 (merged)) is same easy as it was previously, so instead of

diff --git a/values.yaml b/values.yaml
index c15901e..2921569 100644
--- a/values.yaml
+++ b/values.yaml
@@ -1,7 +1,7 @@
 ## GitLab Runner Image
 ## ref: https://hub.docker.com/r/gitlab/gitlab-runner/tags/
 ##
-image: gitlab/gitlab-runner:alpine-v11.5.1
+image: gitlab/gitlab-runner:alpine-v11.6.0

it would be:

diff --git a/Chart.yaml b/Chart.yaml
index 501f3c5..2ff951a 100644
--- a/Chart.yaml
+++ b/Chart.yaml
@@ -1,5 +1,6 @@
 name: gitlab-runner
 version: 0.1.43
-appVersion: 11.5.1
+appVersion: 11.6.0
 description: GitLab Runner
 keywords:
 - git