Add extract_repos_subset.py script

[scpeters]

Add script to extract a subset of repository information from a .repos yaml file.

$ curl https://github.com/ignition-tooling/gazebodistro/raw/master/collection-fortress.yaml
...
$ python3 jenkins-scripts/tools/extract_repos_subset.py collection-fortress.yaml sdformat > fortress_sdformat.repos
jenkins-scripts/tools/extract_repos_subset.py:22: YAMLLoadWarning: calling yaml.load() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.
  y = yaml.load(f.read())

$ cat fortress_sdformat.repos
repositories:
  sdformat:
    type: git
    url: https://github.com/osrf/sdformat
    version: sdf12

I needed this script for VIPER, so I figured I would post it here in case others find it useful.