Following on from discussion in #51, we should explore the possibility of supporting a default Issuer.

We should look to follow a similar pattern as used for StorageClass, namely:

• A default Issuer can be set by putting a certmanager.k8s.io/is-default annotation on an Issuer.
• If a Certificate is created without the Issuer field set, the default Issuer should be chosen and set by the Initializer.
  • If there is no default issuer set, do nothing
  • If there is one default issuer set, set the Issuer field to the name of this Issuer
  • If there is more than one default issuer set, reject the request and log an error on the Issuer

This issue does not deal with default Issuer configuration (e.g. automatically selecting a challenge mechanism to use with the ACME issuer).

/cc @mikebryant