conditional usage of q2galaxy to support build with tiny distribution

[gregcaporaso]

Currently trying to build the docs in a tiny distribution environmental fails because q2galaxy isn't available:

Extension error:
Could not import extension q2doc.usage (exception: No module named 'q2galaxy')
make: *** [Makefile:48: preview] Error 2

We should update this so that building usage examples for galaxy is conditional on q2galaxy being available, as this would enable faster development on the docs (no plugin pages will be built for the tiny distro, and this takes a lot of time right now).