tox.ini: add sphinx-build flags for strictness
tox.ini: add sphinx-build flags for strictness

The following flags are now specified when docs are built:
    -n: nitpicky mode
    -W: turn warnings into errors

See http://sphinx-doc.org/invocation.html for more info.

zerovm/zpm,zerovm/zerovm-cli,zerovm/zerovm-cli,zerovm/zpm,zerovm/zpm,zerovm/zerovm-cli,zerovm/zerovm-cli,zerovm/zerovm-cli,zerovm/zerovm-cli,zerovm/zpm,zerovm/zpm,zerovm/zpm