Fixes scanners' invocation in scan stage in weekly scan template

[navidshaikh]

This changeset fixes the way we invoke the scanners.

• Adds --user root option to docker run command. This fixes yum updates scanner to have right permission while performing yum related tasks.

• Updates the absolute path of python binary in entrypoint option to /usr/bin/python. This makes the scanners compatible to run against RHEL6 based images.

• References the $image_name_with_registry while invoking the scans. Earlier $image_name was referenced - which could be pulling the same image again from configured registry, beside it has already pulled the image in earlier stage.

[navidshaikh]

@bamachrn : Looking forward to your review on this, we'll need this fix in for upcoming weekly scan.