Description of problem:
When use command `oadm verify-image-signature sha256:67326d0fcdc02a022a7b507b832b76a8609b8246de926ee0fa12f64886da1803 --expected-identity=172.30.205.231:5000/zhouy/hello-openshift:latest --remove-all`, the signature was not moved from the image object.

Version-Release number of selected component (if applicable):
openshift v3.6.74
kubernetes v1.6.1+5115d708d7
etcd 3.1.0

How reproducible:
always

Steps to Reproduce:
1. Login in Openshift and create project;
2. Create a GPG Key Pair;
3. Use the user token to login the integrated docker-registry;
4. Tag a image to integrated docker-registry:
   `docker tag openshift/hello-openshift   docker-registry:5000/project/imagestream:latest`
5. Use the atomic command to sign and push the image to integrated docker-registry:
   `atomic push --type atomic --sign-by yinzhou 172.30.205.231:5000/zhouy/deployment-example:latest`
6. Check the image with signature;
7. Use command to remove the signature:
    `oadm verify-image-signature sha256:3f71e6dd452146b942f064d3aa4a8f33e7993aaae4e9edc691a638089944093f  --expected-identity=172.30.205.231:5000/zhouy/deployment-example:latest --remove-all`
8. Use command to remove the signature:
   `oadm verify-image-signature sha256:3f71e6dd452146b942f064d3aa4a8f33e7993aaae4e9edc691a638089944093f  --expected-identity=172.30.205.231:5000/zhouy/deployment-example:latest  --remove-all --public-key=/root/.gnupg/pubring.gpg`


Actual results:
7. The command will failed with error: unable to read --public-key: open pubring.gpg: no such file or directory
8. No error , but the '--remove-all' option does not work. 


Expected results:
7. When use '--remove-all' option should not work with '-public-key' option;
8. Should remove the signature succeed.


Additional info: