Add route subdomain support

[mbaldessari]

This change adds route subdomain support (documented at [1]). One of the
reasons for adding this is that setting a subdomain requires no
knowledge at all of the global ingress domain of the cluster.
Since kubernetes has no concept of cluster name [2] and proposals to
change that have gone nowhere [3], we add subdomain support which allows a user
to simply set the server.route.subdomain value which will be then used
to construct the FQDN by adding the ingress cluster DNS domain. (E.g.
server.route.subdomain: foo will generate the FQDN "foo.apps.mycluster.com")

An additional reason for this change is that helm does not currently
work when setting 'null' values when using subcharts [4].

The status quo of having a default value in server.route.host has not
been changed and setting a subdomain will take precedence over the host
value.

Tested successfully on a 4.11 OCP cluster (mycluster.com) with the
following values:

---
global:
  openshift: true

injector:
  enabled: false
ui:
  enabled: true
  serviceType: "LoadBalancer"
server:
  route:
    enabled: true
    subdomain: "foobar"
    tls:
      termination: "edge"
  image:
    repository: "registry.connect.redhat.com/hashicorp/vault"
    tag: "1.11.2-ubi"

and correctly obtained a route that pointed to foobar.apps.mycluster.com

[1] https://docs.openshift.com/container-platform/4.6/rest_api/network_apis/route-route-openshift-io-v1.html
[2] kubernetes/kubernetes#44954
[3] https://docs.google.com/document/d/1F__vEKeI41P7PPUCMM9PVPYY34pyrvQI5rbTJVnS5c4/edit
[4] helm/helm#9136

[mbaldessari]

Now that https://www.github.com/helm/helm/issues/9136 is fixed, I think this is less interesting. Closing.