stack/lamp: use python3 at lamp stack level

This changes software/nextcloud to be python3 and software/matomo no
longer need to explicitly use python3 anymore.

software/matomo/software.cfg

@@ -17,10 +17,6 @@ parts =
 # to create file instance.cfg of all instances
   instance
 
-#to make sure we use python3
-[python]
-part = python3
-
 # download bas
 # inherited by modules that need to download files
 [matomo-download]

stack/lamp/buildout.cfg

@@ -38,6 +38,9 @@ url = ${:_profile_base_location_}/${:filename}
 [application]
 recipe = slapos.recipe.build:download-unpacked
 
+[python]
+part = python3
+
 #----------------
 #--
 #-- Optional part allowing applications using this stack to run a custom