fix: fix keymanager api for prysm

[TobiWo]

Summary

This PR fixes the keymanager api feature for prysm which was not working. In detail it:

• delete the unknown http flags and replaces them with the grpc-gateway flags since all REST APIs are exposed as gateway to the general grpc protocol
• deletes the unused/unnecessary grpc port for the public port exposure

[barnabasbusa]

Lets wait for this PR to be merged once this is in stable prysm image.

[barnabasbusa]

Changing the default prysm img to point at latest stable instead of latest develop: #793

[barnabasbusa]

Actually, it looks like prysm stable branch is still using grpc-gateway, and latest develop uses http.

You might have had some older version of the prysm image locally.

[barnabasbusa]

is this pr still needed?
Prysm made a new stable release that does http instead of grocery.

no longer needed pr

[TobiWo]

Please keep it open for now. I just need it as reference because I think there is still a bug in the prysm config (not related to grpc). I will test in the next days and will close it once I did all testing.

[TobiWo]

Closed as described here.