Update the 'fullname' template to match helm 2.8.2
Update the 'fullname' template to match helm 2.8.2

The newer 'fullname' helper function automatically avoids name duplication. This change updates the function in cert-manager to include that functionality.

Given there is generally only one cert-manager per cluster, if you install cert-manager and call it `cert-manager`:
```
helm upgrade --install cert-manager stable/cert-manager
```
You get deployments and pods called `cert-manager-cert-manager...` 

With the update  version (taken from the `helm create` template for helm 2.8.2` you instead get deployments and pods called `cert-manager`.

There is a workaround possible at the moment, but the automatic behavior is more intuitive for new users.
```
helm upgrade --install cert-manager stable/cert-manager --set=fullnameOverride=cert-manager
```
cert-manager/cert-manager,cert-manager/cert-manager,jetstack/cert-manager,cert-manager/cert-manager,jetstack-experimental/cert-manager,jetstack/cert-manager,jetstack/cert-manager,jetstack-experimental/cert-manager,jetstack-experimental/cert-manager