Mesh: Refactor Network group Removal logic

Mesh: Refactor Network group Removal logic

Network Groups (Mesh group & virtual Mesh group)
are created and saved in Network Configuration DB.
Therefore, removal of groups by user should remove these
groups from Conf DB also, instead of just removing handles
from application.

Apart from above, this patch refactors 'Get All Mesh groups'
logic. Instead of just fetching the group address, now it fetches
all informations of groups (Label UUID, if Virtual group)

Change-Id: I8ca483ddf46140c3e671070f41a6484b5a04935e
Signed-off-by: Anupam Roy <anupam.r@samsung.com>