HOST env var is clobbered by conda and prevents using run_server without overiding host explicitly

[schaffino]

I noticed a peculiar issue when trying to use jupyter-dash wherein it would try and connect to x86_64-conda_cos6-linux-gnu when execute app.run_server.

It seems that when a conda environment is activated, it modifies the HOST variable (a very strange idea, but one that has been noted and not fixed for a number of years).

As i imagine jupyter-dash is used very often with conda, could i suggest that HOSTNAME is used instead of HOST if it exists as this isn't clobbered by conda activation. Or is there any reason that host shouldn't just default to 127.0.0.1 unless explicitly set, given how the plugin operates?