Is your feature request related to a problem? Please describe.
I would like to have a way to use the ingress shim annotations, but I also need to set the key size, since I have a Vault PKI backend that does not allow the default key size of 2048.

Describe the solution you'd like
Either: Additional annotations to set on the ingresses
Or: Custom defaults for key size and algorithm e.g. on the Issuer resource

The custom defaults would be my favourite and would benefit not only the ingress shim situation.

Describe alternatives you've considered
An admission controller that changes the certificate resources that are created by the cert-manager.

Additional context
none

Environment details (if applicable):

• Kubernetes version (e.g. v1.10.2):
• Cloud-provider/provisioner (e.g. GKE, kops AWS, etc):
• cert-manager version (e.g. v0.4.0):
• Install method (e.g. helm or static manifests):

/kind feature