fix: Add Packer 1.11.x plugin compatibility

fix: Add Packer 1.11.x plugin compatibility

- Added required_plugins blocks to all template files
- Declared openstack, docker, amazon, and ansible plugins
- Added .packer.d/ to .gitignore

Fixes compatibility with Packer 1.11.x while maintaining
backward compatibility with 1.9.x and 1.10.x.

Templates updated:
- builder.pkr.hcl
- docker.pkr.hcl
- devstack.pkr.hcl
- devstack-pre-pip-yoga.pkr.hcl
- windows-builder.pkr.hcl
- builder-aws.pkr.hcl
- docker-aws.pkr.hcl

Issue: RELENG-5871
Change-Id: Id7c980a76f80bfff011fea5a53c624d83cbbab2d
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>