Let tools/drake_visualizer work outside the sandbox

[jwnimmer-tri]

This fixes a few loose ends from #6259:

• Let tools/drake_visualizer work outside the sandbox
• Tidy up drake-visualizer launching in READMEs

All three of these methods have been tested to work:

bazel build //tools:drake_visualizer && ./bazel-bin/tools/drake_visualizer 
bazel run //tools:drake_visualizer 
bazel run //drake/automotive:demo

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This change is 

[ggould-tri]

+@ggould-tri

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Review status: 0 of 3 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[jamiesnape]

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Reviewed 3 of 3 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[ggould-tri]

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Reviewed 3 of 3 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[patmarion]

By the way, this might indicate an issue with the precompiled drake-visualizer package. In the past, drake-visualizer has never required the user to set LD_LIBRARY_PATH or PYTHONPATH.

[jamiesnape]

We are in a sandbox now and we are not using system installs of VTK.

[jwnimmer-tri]

Only non-Bazel builds remain. Using override-merge.