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.
Previously, each provider's package-level 'New' function was being
called.
That made mocking it out for a different function that records data or
returns different output quite difficult.
This PR introduces an additional layer of abstraction in the form of
effectively a vtable struct for the dns providers. It's defaulted to the
same package-level constructors as before, but unit tests in the dns
package can easily override it.
A new test for the previously-introduced route53 trimming behavior is
also added.
