grid: fix distribution_tuple()

 * consider raspbian as a debian
 * never set id

After the change to distro.linux_distribution() in
425f5bb7, raspbian is correctly detected
and id is set, see the execution of both functions on raspbian:

>>> platform.linux_distribution('', '', '', platform._supported_dists, 1)
('debian', '10.11', '')

>>> distro.linux_distribution(full_distribution_name=False)
('raspbian', '10', 'buster')

slapos/grid/distribution.py

@@ -57,4 +57,10 @@ def os_matches(os1, os2):
 
 
 def distribution_tuple():
-    return distro.linux_distribution(full_distribution_name=False)
+    distname = distro.id()
+    version = distro.version()
+    if distname == 'raspbian':
+        distname = 'debian'
+    # we return something compatible with older platform.linux_distribution()
+    # id (last field of the tuple) was always empty
+    return distname, version, ''