Acquiring nodes should be made easier to the user

[eduble]

Node owership and how to acquire nodes may seem complicated at first for the new user. It should be improved.

Currently, walt node create creates virtual nodes with their default image waltplatform/<node-type>-default, thus they appear as free nodes and they do not belong to the user who creates them. They are hidden when user types walt node show without --all option, which is quite unexpected.
This could be changed by associating these new virtual nodes to image <user>/<node-type>-default instead. If this image is missing, it would be automatically cloned from default image waltplatform/<node-type>-default.

A command walt node acquire <node-set> should also be provided.
It would do the following:

• if some nodes of <node-set> belong to another user (i.e. a real user, not waltplatform), ask confirmation, and if yes, continue.
• if some nodes of <node-set> already belong to current user, a warning would be printed and these nodes would be ignored in the following step
• selected nodes would be associated to image <user>/<node-type>-default (cloned if missing, as for virtual nodes above) and then rebooted.

A keyword free-nodes could be introduced, to allow for instance walt node acquire free-nodes.

Help message of walt node show when user has no nodes yet should be updated to propose walt node acquire.

walt node show could have another option --free to show free nodes only, and not the nodes belonging to other users.

A command walt node release <node-set> could also be proposed as an alias to walt node boot <node-set> default.