Add example of PackageMap and an ament resource

[EricCousineau-TRI]

Condensed, but less functional, version of #1

FYI @adeeb10abbas @jwnimmer-tri This example won't pass on CI or w/ Bazel archive, but should work if you use a package resource that actually exists.
In my case, I dumpster dove to find the test URDF. Better solution is to vendor our own.

Relates ament changes in #133

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This change is 

[EricCousineau-TRI]

Reviewable status: 0 of 4 files reviewed, 1 unresolved discussion (waiting on @EricCousineau-TRI)

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drake_ros_examples/test/parse_ros_model_test.py line 24 at r1 (raw file):

    # TODO(eric.cousineau): Use a non-test resource.
    parser.AddModelsFromUrl(
        "package://rviz_default_plugins/test_meshes/test.urdf"

Working: This needs a real package. Ideally, I pull in drake_example_pendulum from Addisu's example.

[EricCousineau-TRI]

Reviewable status: 0 of 4 files reviewed, 2 unresolved discussions

a discussion (no related file):
Working: Should wait until #133 lands.

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