Management API : groups of gateway related methods

[jbtrystram]

The gateways groups feature requires to add a few API calls. In a UI workflow, when creating a devices, the user can enter the ID of a gateway group for via-group or member-of.

Here is a screenshot of a design draft : https://marvelapp.com/4g344f5/screen/69036293

However, the there is no API function to retrieve a list of the existing gateway groups, which makes the problem of having a device not connecting because of a typo in the "via-group" field very plausible.

Doing a getAll operation on the devices and parsing the data to build a group list would be very expensive, so I suggest to add a new endpoint to the api. It could be called "groups" and to manage groups of gateways, such as creating groups, get all devices in a group, and delete groups.

Also, an extra endpoint doesn't break the existing API.