This implements ambient credential support for AWS, gated behind flags
for issuers and cluster issuers.
This adds the pair of flags discussed in
https://github.com/jetstack/cert-manager/issues/308.
It provides an implementation for those flag's effects for the route53
solver.
The zone id is never read from the environment; this test tests
functionality which doesn't exist in the actual software, so there's no
point in having it.
This change maintains backwards compatibility, but makes the `accessKeyID` and `secretAccessKeySecretRef` fields of the `route53` DNS provider optional.
If not provided, AWS credentials will be loaded from `AWS_*` environment variables or the EC2 metadata service.
This should also work for things that impersonate the EC2 metadata service, such as [kube2iam](https://github.com/jtblin/kube2iam) and [kail](https://github.com/uswitch/kiam).
Signed-off-by: Matt Moyer <moyer@heptio.com>
