Add 'solvers' configuration to ACME Issuer

[munnerz]

What this PR does / why we need it:

This is a very WIP first pass at moving the ACME certificate config over to the Issuer resource in a backwards compatible manner.

We'll remove support for the old syntax at {some point} (either pre-1.0, or for 1.0).

Mostly pushing this up to see if the e2e tests (which have not changed) will pass 😄

A (very verbose) example of an Issuer using a mixture of new and old configuration, and also including an example of how the 'webhook' code in #1443 can be adapted for HTTP01:

apiVersion: certmanager.k8s.io/v1alpha1
kind: Issuer
metadata:
  name: letsencrypt-staging
spec:
  acme:
    server: ...
    privateKeySecretRef:
      name: le-pk
    solvers:
    # use dns01 by default
    - selector: {} # match all certificates
      dns01:
        # TODO: should we take this opp. to remove support for 'core' solvers?
        webhook:
          groupName: solvers.acme.cert-manager.io
          solverName: cloudflare
          config:
            email: james@munnelly.eu
            apiKeySecretRef:
              name: cloudflare-api-key
              key: api-key
    - selector:
        matchLabels:
          "use-acme-http01-nginx": "true"
      http01:
        webhook:
          groupName: solvers.acme.cert-manager.io
          solverName: ingress
          config:
            ingressClass: nginx
    - selector:
        matchLabels:
          "use-acme-http01-contour": "true"
      http01:
        webhook:
          groupName: solvers.acme.cert-manager.io
          solverName: contour
          config:
            # config required for contour http01 solver,
            # e.g. name of IngressRoute
            ingressRouteName: my-root-ingress-route
    # Example showing backwards compatibility with non-webhook types
    # TODO: do we want to ever support the old format here?
    - selector:
        matchLabels:
          "use-acme-http01-nginx-non-webhook-format": "true"
      http01:
        ingressClass: "nginx"
    dns01:
      providers:
      - name: "old-legacy-format-provider"
        cloudflare:
          email: james@munnelly.eu
          apiKeySecretRef:
            name: cloudflare-api-key
            key: api-key
    http01:
      serviceType: NodePort

Which issue this PR fixes: fixes #1342

Special notes for your reviewer:

Release note:

TODO

/cc @DanielMorsing

[munnerz]

/cc @Evesy this field currently only supports exact matches, FYI

[munnerz]

/cc @kragniz should these two fields be Class and Name to avoid the stutter?

[munnerz]

Updated to Name and Class 😄

[kragniz]

One question and a couple of suggestions, happy to merge

[kragniz]

/lgtm

[jetstack-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kragniz, munnerz

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [kragniz,munnerz]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment