Ansible request for linux_installer role

Please put the name of the software product (and affected platforms if relevant) in the title of this issue

- [ ] Missing install
- [ ] Bug in ansible playbook
- [x] Request for new playbook addition

Details: Currently the [linux_installer role](https://github.com/AdoptOpenJDK/openjdk-infrastructure/blob/master/ansible/playbooks/AdoptOpenJDK_Unix_Playbook/roles/linux_installer/tasks/main.yml) as called from [linux_installer_setup.yml](https://github.com/AdoptOpenJDK/openjdk-infrastructure/blob/master/ansible/playbooks/AdoptOpenJDK_Unix_Playbook/linux_installer_setup.yml). For this to work in practice it is necessary for an `.rpmmacros` file to be available to jenkins as per the end of [this section](https://github.com/AdoptOpenJDK/openjdk-installer/tree/master/linux#upload-to-package-repositories) of the docs, and for `.gnupg` to be populated.

Since the output of the playbook will not be usable without this file, we should add a final step that checks for the presence of it and if not, prints out a warning to the user to notify them that the extra step is required.

Start with ansible/playbooks/AdoptOpenJDK_Unix_Playbook/roles/linux_installer/tasks/main.yml and its use from linux_installer_setup.yml. Review the linked installer documentation for the required .rpmmacros and .gnupg setup. Done means the playbook checks for the required file and warns the user when the extra setup is missing.

Written by the indexing model from the issue text.