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: I134a9b7dc2366eeb7d07d3ae07deecdd173cb4a6

snapcraft.yaml

@@ -9,23 +9,27 @@ description: |
 confinement: classic
 grade: devel
 
-environment:
-  PATH: $PATH:$SNAP/bin/
-
 apps:
   api:
-    command: snap-openstack glance-api
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack glance-api
     daemon: simple
   registry:
-    command: snap-openstack glance-registry
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack glance-registry
     daemon: simple
   manage:
-    command: snap-openstack glance-manage
+    command: >
+      env PYTHONPATH=$PYTHONPATH:$SNAP/lib/python2.7/site-packages
+      $SNAP/usr/bin/python2 $SNAP/bin/snap-openstack glance-manage
     aliases:
       - glance-manage
 
 parts:
   glance:
+    after: [python]
     plugin: python
     python-version: python2
     source: http://tarballs.openstack.org/glance/glance-master.tar.gz
@@ -44,6 +48,11 @@ 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 $SNAPCRAFT_PART_INSTALL/lib/python2.7/site-packages/repoze/__init__.py
@@ -67,3 +76,16 @@ parts:
         - etc/glance/*.json
     stage: [$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