Remove the sed variable overrides from the build job and move them to PKR_VAR_extra_vars

Description

Some ansible variables (refer here) like secrets are defined/overridden in packer builds during runtime. They are not defined in ansible/group_vars in plain text for obvious reasons. We used sed to define the values during runtime but they can be defined as extra_vars for packer. This will enable us to maintain clean code.

Proposed solution

Remove the ansible var overrides in .gitlab-ci.yml in build_proxy_image and build_ood_image jobs. Define these variables in schedule pipeline via PKR_VAR_extra_vars