Creating a second slice replaces default device group

[markbeierl]

Describe the bug

Default device group is overwritten every time a slice is created

To Reproduce

1. Create a slice
2. Delete the default device group
3. Create a new default device group with different values
4. Confirm new values are present
5. Create a new slice
6. Reload the page and note the reset in slice 1's default

Expected behavior

Screenshots

First slice Second slice, both defaults are set back to initial values

[patriciareinoso]

After https://github.com/canonical/sdcore-nms/pull/226 the default Device Group is no longer called default. So it should help with this issue. I agree we should check whether the DG exist before creating it. It's in the backlog

[gruyaume]

Closing this issue as this was addressed in #226