Refactor install_tensorflow

[dfalbel]

Delegates installation to reticulate functions.

I have set testing for different envs and OS's here: https://github.com/dfalbel/tfinstall

[javierluraschi]

LGTM!

[skeydan]

To me too, great! Just asking about d71d12f, when I create a default new environment

conda create -n new python=3.7

I get these automatically:

-rw-rw-r--.  5 key key      132 Jun  9 22:30 libgcc_s.so
-rw-rw-r--.  5 key key   818984 Jun  9 22:30 libgcc_s.so.1

[dfalbel]

You are right! I fixed this here: 4c58d7e#diff-b5f7f6437b8e3c21cb2e9d6eb0ce1dd8L138

I added a conda_python_version argument to control which python version is installed in this env. Currently using 3.6.8 since 3.7 has some ABI incompatibility issues with tensorflow.