clusterctl: Allow set ImageRepository and ImageTag for certmanager

[fabriziopandini]

User Story

As an operator I would like to use my own image repository for the cert-manager

Detailed Description
The cert-manager manifest is embedded in clusterctl so it cannot be easily changed like e.g. provider manifests. So configuration flags are required to allow to set imageRepository/ImageTag (similar to ImageMeta for etcd/CoreDns in kubeadm)

Anything else you would like to add:
I prefer to use the config file vs flags because it makes the configuration persistent and it ensures consistency between init and upgrades.
The configuration can be something similar (only the image overrides part):

# custom providers
providers:
  - name: docker
    url: /Users/fpandini/.cluster-api/overrides/infrastructure-docker/latest/infrastructure-components.yaml
    type: InfrastructureProvider

# Values for env variables substitutes in provider assets
AWS_B64ENCODED_CREDENTIALS: xxxx

# image overrides
images:
  cert-manager:
    repository: us.gcr.io/myRepo/cluster-api
    tag: v0.13.5 

This can easily evolve into

images:
  all:
    repository: us.gcr.io/myRepo/cluster-api
  cert-manager:
    tag: v0.13.5 

Supporting overrides for all the components or into supporting more granular type of images mutation if the needs arise

/kind feature
/cc @vincepri
/cc @ncdc

[vincepri]

I'm fine using the configuration file if that's easier, the example you posted looks good to me

/milestone v0.3.0

[ncdc]

config file ++ LGTM thx 😄

[ncdc]

/lifecycle active