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: I5505bb19b026589c94df4c2753a7ee4b7e2f8fb1

snapcraft.yaml

@@ -17,28 +17,36 @@ description: |
 confinement: classic
 grade: devel
 
-environment:
-  PATH: $PATH:$SNAP/bin/
-
 apps:
   api:
-    command: snap-openstack nova-api-os-compute
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack nova-api-os-compute
     daemon: simple
   conductor:
-    command: snap-openstack nova-conductor
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack nova-conductor
     daemon: simple
   scheduler:
-    command: snap-openstack nova-scheduler
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack nova-scheduler
     daemon: simple
   consoleauth:
-    command: snap-openstack nova-consoleauth
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack nova-consoleauth
     daemon: simple
   manage:
-    command: snap-openstack nova-manage
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack nova-manage
     aliases:
       - nova-manage
 parts:
   nova:
+    after: [python]
     plugin: python
     python-version: python2
     source: http://tarballs.openstack.org/nova/nova-master.tar.gz
@@ -51,6 +59,11 @@ 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 $SNAPCRAFT_PART_INSTALL/lib/python2.7/site-packages/repoze/__init__.py
@@ -70,3 +83,16 @@ parts:
         - etc/nova/rootwrap.d
     stage: [$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