Misc updates from SSL testing (#14)

Drop build of libxml2 (picked from distro).

Use NGINX 1.12.x series (as this is the stable version).

Use constraints file from OpenStack Pike due to transient
dependency bumps on PyPI causing build failures.

.gitignore

@@ -12,3 +12,4 @@ __pycache__
 # emacs
 *~
 \#*
+*.bz2

snapcraft.yaml

@@ -44,6 +44,7 @@ parts:
     plugin: python
     python-version: python2
     source: https://pypi.python.org/packages/31/04/f4a0d82279a9be0638e0f79a141e2f6295b18ad6c65b53ef716625fa712f/gnocchi-4.0.3.tar.gz
+    constraints: https://raw.githubusercontent.com/openstack/requirements/stable/pike/upper-constraints.txt
     python-packages:
       - futurist
       - keystonemiddleware
@@ -92,7 +93,7 @@ parts:
     stage: [$etc]
     prime: [$etc]
   nginx:
-    source: http://www.nginx.org/download/nginx-1.13.0.tar.gz
+    source: http://www.nginx.org/download/nginx-1.12.2.tar.gz
     plugin: autotools
     configflags:
       - --prefix=/usr
@@ -113,6 +114,3 @@ parts:
       export SNAP_ROOT="../../.."
       export SNAP_SOURCE="$SNAP_ROOT/parts/nginx/build"
       patch -d $SNAP_SOURCE -p1 < $SNAP_ROOT/patches/drop-nginx-setgroups.patch
-  libxml2:
-    source: http://xmlsoft.org/sources/libxml2-2.9.4.tar.gz
-    plugin: autotools