fix(controller): agent image name parsing (backport #42)

[mergify]

IMPORTANT: Please do not create a Pull Request without creating an issue first.

Problem:

Previously, enabling the Managed DHCP add-on with images containing port numbers in their name caused the controller pod to crash.

Solution:

Handle the image name properly with support for the following cases:

• Names containing port numbers
• Names without tags (default to latest)

Related Issue:

harvester/harvester#7549

Test plan:

1. Prepare a v1.4.1 Harvester cluster
2. Create the Managed DHCP Addon CR with the following content

   apiVersion: harvesterhci.io/v1beta1
   kind: Addon
   metadata:
     name: harvester-vm-dhcp-controller
     namespace: harvester-system
     labels:
       addon.harvesterhci.io/experimental: "true"
   spec:
     enabled: false
     repo: https://charts.harvesterhci.io
     version: 0.3.3
     chart: harvester-vm-dhcp-controller
     valuesContent: |
       image:
         repo: starbops/harvester-vm-dhcp-controller
         tag: fix-7549-head
       agent:
         image:
           repo: myregistry.local:5000/rancher/harvester-vm-dhcp-agent
   

3. Enable the add-on
4. The controller manager pod should be running fine without crashing

   ---

   This is an automatic backport of pull request fix(controller): agent image name parsing #42 done by Mergify.