Add tests for image signatures

[mfojtik]

Currently the image signature code in origin is not covered by any tests except manual QE testing.
We should figure out how to:

1. Push a signature using "skopeo copy" tool and verify the signature was created successfully
2. Push a signature using "curl" command directly using the registry signature endpoint.
3. Get the signature using "curl" command from signature endpoint.
4. Remove signature from the image
5. Verify the image signature

(the 4) and 5) has to be done after #13585 merges).

[dmage]

@stevekuznetsov how can I use skopeo in our end-to-end tests?

[stevekuznetsov]

Depends on how you want to use it. I would love to not add more Bash-based test suites unless we need to. @mfojtik what is the requirement here? Can we get the Skopeo source on there and use their source in compiling Go-based tests? Do we need to install them as an RPM and use them on the command line?

[dmage]

Now we have unit tests TestSignatureGet and TestSignaturePut.