Use slapos.recipe.build:download-unpacked for Python 3 support

hexagonit.recipe.download-1.7.post4 (Nexedi's patch) is not compatible
with Python 3 (yet?).

component/nginx/buildout.cfg

@@ -30,8 +30,7 @@ configure-options=
   --with-cc-opt="-I ${openssl-1.0:location}/include -I ${pcre:location}/include -I ${zlib:location}/include"
 
 [nginx-dav-ext-module]
-recipe = hexagonit.recipe.download
-ignore-existing = true
+recipe = slapos.recipe.build:download-unpacked
 url = https://github.com/arut/nginx-dav-ext-module/archive/v0.0.3.tar.gz
 strip-top-level-dir = true
 md5sum = 2cb502dbda335be4ebd5fed0b3182bae

component/noVNC/buildout.cfg

@@ -3,8 +3,7 @@ parts =
   noVNC
 
 [noVNC]
-recipe = hexagonit.recipe.download
-ignore-existing = true
+recipe = slapos.recipe.build:download-unpacked
 # version-1.0.0 release on Feb 22, 2018
 url = https://github.com/novnc/noVNC/archive/v1.0.0.tar.gz
 md5sum = d63c2944abd12fecec6ec504e82f27c8