Switch elb/ec2 over to signature version 4

This matches the public documentation and other AWS SDKs, which
all use signature version 4 for these services.

Note that this is no longer a detect_potential_sigv4, we should be
using signature version 4 at all times for ec2/elb.

I've also updated an assertion method to not require that the
ignore_params_values actually exist in the request.  We made
a similar change in the AWS CLI a while back.  All we care about is
that we're not validating those params.  We don't actually care
if they're in the serialized params are not.

Unittests pass, also manually tested in the REPL.


cc @danielgtaylor