Global backup definition option enhancements

[klebed]

1. Most important part of the feature request is in setting default bandwith cap for backup definitions. Fullspeed backups may affect node performance. Also backups may intersect in time, so it might make things even worse. Default bandwith limit could solve such problems.

So first suggestion is to make global/domain backup bandwith limit option. And make it easier by enable typing M and G, like in RAM/Disk size fields.

2. Also it might be worth making global backup map to even backup load and see intersections. Backup time might be approximately suggested based on disk sizes and throughputs. But it will bring an overview over all backups in whole infrastructure.

3. Also it might be useful to make a default backup definitions for every machine created. It might be disabled/enabled by default but it will make easier to manage backups.

[YanChii]

Good idea. The first part is probably not hard to do. Would you like to try to add it? This is an OSS so we all improve this project by contributions. This PR is a nice example of adding a new global configuration parameter. And then you just add the appropriate logic to the place where the backup command is constructed.

The third point can be achieved also by VM templates. You can choose what you want to have defined in a template. It can be only a backup definition.

The second one is a bit more tricky. Not sure about the implementation. But all the neccesary data is in the pg DB.

Jan