Implement temporary resources cleanup

See merge request !36

The Docker and subprocess executors use one or more
temp folders during processing. This patch implements
proper removal of them when they are done.

There's a debug settings that can be used to switch
that cleanup off.

Docker improvements

See merge request !35

This by default disable tests that requires the capabilities
to create new docker network on the fly.

Explicitly set the name of the container and retrieve its
address with the new Host API.

Currently only used by docker executor.

Now allows to use nfs

Also refactor related tests to avoid spending time discovering
environments when the test doesn't need it.

Allows to speed up manipulation if the environment is already
known and available.

Disable python2 builds

Closes #61

See merge request !34

Merge development branch 1.6.x

Closes #47, #54, #55, and #51

See merge request !32