Add support for different RHEL versions

We need to add an extra parameter to the integrate job so that it can perform PR integration for jobs w/ RHEL versions other than 7.

Example of a failed job: https://fusesource-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/view/Productization/job/productization-rd-integrate-PRs-into-productization-branch/23/console

Steps:

1. Modify the container-merge-prs-into-productization-branch.sh to receive an extra parameter
2. Replace all occurrences of rhel-7 with rhel-${RHEL_VERSION}
3. Modify the job to add one additional parameter RHEL_VERSION
4. Include that parameter in the CLI that calls the container-merge-prs-into-productization-branch.sh