Update gitaly service name templating through the chart.

Fixes #1712 (closed)

The gitaly templates did not render the configured gitaly service name into the configurations in the way one would expect. This merge request fixes that issue:

• It creates a template gitlab.gitaly.serviceName which checks, in order:

  1. .Values.gitaly.serviceName, default unset
  2. .Values.global.gitaly.serviceName, default set to gitaly
  3. template "fullname" .

  This is then used instead of the "fullname" template in the gitaly service spec, and the gitaly statefulset service.

• It updates the gitaly configuration snippet template to use the above-named template, together with a shim that emulates the "fullname" template as called from another chart.
  This shim was nescessary because helm has chart value accessibility constraints which are difficult to impossible to work around. This is one of the more elegant methods in my opinion.

It also moves the configuration (with deprecation warning) of gitaly's Service's name has been moved from .Values.serviceName in that chart's scope, to the global global.gitaly.serviceName configuration, with a chart-local override option. This allows all charts to access this value, and makes the templating in all charts easier.