g5k: Don't deploy by default, and require an env_name when deploying

The idea is to make things more explicit and reproducible:

• by default (no job_type), we now use the standard Grid'5000 env with sudo-g5k instead of deploying. This is much faster to provision, and matches the Grid'5000 oarsub default when no job type is specified.

• when using the deploy job type, we now also require the env_name parameter instead of deploying a default env (currently debian11-nfs). This makes sure that the user explicitly chooses an environment to deploy, which is good for reproducibility: the default env chosen by Enoslib tended to evolve over time as new stable Debian releases become available on Grid'5000.

For users that don't specify any job type, they will now end up with a debian11-std environment instead of debian11-nfs. This is very similar, except that many more software packages are installed ("std" is equivalent to "big" in [1]).

For backwards compatibility, we automatically add the 'deploy' job type if the user specifies an env_name (to account for the previous default of deploying). However, user code should be updated.

Also for backwards compatibility, if a Kavlan network is used, we automatically add the 'deploy' job type and set env_name to "debian11-nfs" if missing. But again, user code should be updated to be more explicit and choose an appropriate environment to deploy.

Note that "allow_classic_ssh" is still supported as job type but has no effect on Grid'5000 and is deprecated (see #130 (closed)). You can get the same behaviour by just using job_type=[] or omitting the job type altogether. This is because of a recent change in the Grid'5000 platform itself that made allow_classic_ssh the default behaviour.

[1] https://www.grid5000.fr/w/Getting_Started#On_Grid.275000_reference_environments