Compile python and set environment variables

Classic python snaps require python to be compiled from source.
Additionally, this change adds environment variables required for
command execution. We'll do this until the environment dictionary
is fully supported by snapd, at which point we can use it instead.
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: I09f552c330b5651105c547842b61b3ae737d61ce

snapcraft.yaml

@@ -20,27 +20,43 @@ grade: devel
 
 apps:
   nova-compute:
-    command: snap-openstack nova-compute
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack nova-compute
     daemon: simple
   nova-api-metadata:
-    command: snap-openstack nova-api-metadata
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack nova-api-metadata
     daemon: simple
   neutron-openvswitch-agent:
-    command: snap-openstack neutron-openvswitch-agent
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack neutron-openvswitch-agent
     daemon: simple
   neutron-l3-agent:
-    command: snap-openstack neutron-l3-agent
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack neutron-l3-agent
     daemon: simple
   neutron-dhcp-agent:
-    command: snap-openstack neutron-dhcp-agent
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack neutron-dhcp-agent
     daemon: simple
   neutron-metadata-agent:
-    command: snap-openstack neutron-metadata-agent
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack neutron-metadata-agent
     daemon: simple
   neutron-ovs-cleanup:
-    command: snap-openstack neutron-ovs-cleanup
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack neutron-ovs-cleanup
   neutron-netns-cleanup:
-    command: snap-openstack neutron-netns-cleanup
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack neutron-netns-cleanup
 parts:
   ipset:
     source: http://ipset.netfilter.org/ipset-6.30.tar.bz2
@@ -84,7 +100,9 @@ parts:
     stage: [$bin]
     snap: [$bin]
   nova:
-    after: [openvswitch]
+    after:
+      - openvswitch
+      - python
     plugin: python
     python-version: python2
     source: http://tarballs.openstack.org/nova/nova-master.tar.gz
@@ -103,6 +121,11 @@ parts:
       - libvirt-dev
     stage-packages:
       - qemu-utils
+    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 $SNAPCRAFT_PART_INSTALL/lib/python2.7/site-packages/repoze/__init__.py
@@ -133,3 +156,16 @@ parts:
         - etc/neutron/*
     stage: [$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