Compile python and drop environment dictionary

Classic python snaps require python to be compiled from source.
Additionaly, move away from using the environment dictionary
until it is fully supported by snapd. Finally, use a fixed python
path in order to get the correct site-specific config.

These changes were recommended in the following bug:
https://bugs.launchpad.net/snapcraft/+bug/1675479

Change-Id: Ibe68ab3a252067421169effd99fc5b14d490136b

snapcraft.yaml

@@ -5,18 +5,20 @@ description: OpenStack Network Service (neutron)
 confinement: classic
 grade: devel
 
-environment:
-  PATH: $PATH:$SNAP/bin
-
 apps:
   api:
-    command: snap-openstack neutron-server
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack neutron-server
     daemon: simple
   manage:
-    command: snap-openstack neutron-db-manage
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack neutron-db-manage
 
 parts:
   neutron:
+    after: [python]
     plugin: python
     python-version: python2
     source: http://tarballs.openstack.org/neutron/neutron-master.tar.gz
@@ -29,6 +31,11 @@ parts:
       - gcc
       - libffi-dev
       - libssl-dev
+    stage:
+      - -usr/bin/2to3
+      - -usr/bin/pydoc
+      - -usr/bin/python2.7
+      - -usr/lib/python2.7
     install:
       touch $SNAPCRAFT_PART_INSTALL/lib/python2.7/site-packages/paste/__init__.py
       touch $SNAPCRAFT_PART_INSTALL/lib/python2.7/site-packages/repoze/__init__.py
@@ -51,3 +58,16 @@ parts:
         - etc/neutron/*
     stage: [$etc]
     snap: [$etc]
+  python:
+    source: https://www.python.org/ftp/python/2.7.13/Python-2.7.13.tar.xz
+    plugin: autotools
+    configflags:
+      - --prefix=/usr
+      - --enable-shared
+      - --enable-unicode=ucs4
+    build-packages:
+      - libssl-dev
+    prime:
+      - -usr/include
+    install:
+      $SNAPCRAFT_PART_INSTALL/usr/bin/python2 -m ensurepip