Implement `enos build <provider> [--options]` to install once and use many times a base image

[msimonin]

Rationale:

1. I don't want the docker images to be pulled every single time I use Enos
2. The same for any dependency pulled from internet (apt, pip...)

Note that, for 1., another solution would be to use a persistent registry or a registry backed by a rbd (ceph on g5k only). These solutions don't seem to be convenient for most users and difficult to setup in a dynamic environment such as G5K.

Action Items:

• [ ] add enos build in cli.py
• [ ] orchestrate the build in tasks.py inspired by https://gitlab.inria.fr/discovery/kenan/blob/1d0bc663ff0a41b9c41e08ea3facbaae1c2c2ac2/kenan/provider/static/task.py#L115-126
• [ ] add a get_build_conf in each enos provider so that the instruction on tasks.py build the complete conf and then call deploy.

[msimonin]

I merged this in EnOSlib : https://gitlab.inria.fr/discovery/enoslib/commit/c66bddbbab3d2316442e1e4a39c88d28048136e4

In short, I suggest to generate a working_dir on the frontend (e.g using the current directory) to ease the use of the build process for the VMonG5K provider.

[msimonin]

I also merged this: https://gitlab.inria.fr/discovery/enoslib/commit/b2ed4b3e33fd3c5cdbf9f3bf65a6c01616472238

So, I suggest to generate strategy=copy so that, for vmong5k, the disk image is directly reusable (without the qemu-img commit step).

[jrbalderrama]

We can also add a post build option to commit the results:

• vagrant package ... for vagrant
• tgz-g5k ... for g5k
• Stop the VM to reuse it safely for vmg5k