Install conntrack from Debian package

Building conntrack from source is failing on ppc64 due to:
"build-aux/config.guess: unable to guess system type". We want to
move as many of the commands to stage-packages as possible to get
the benefit of security team support, so let's move conntrack to
stage-packages as a start.

Change-Id: Icb66d012ca516e55a295da05be59c680879c865e

snapcraft.yaml

@@ -98,13 +98,6 @@ parts:
   bridge-utils:
     source: https://www.kernel.org/pub/linux/utils/net/bridge-utils/bridge-utils-1.6.tar.gz
     plugin: autotools
-  conntrack-tools:
-    source: http://www.netfilter.org/projects/conntrack-tools/files/conntrack-tools-1.4.3.tar.bz2
-    plugin: autotools
-    build-packages:
-      - libnetfilter-cttimeout-dev
-      - libnetfilter-cthelper0-dev
-      - libnetfilter-queue-dev
   iproute2:
     source: https://www.kernel.org/pub/linux/utils/net/iproute2/iproute2-4.9.0.tar.gz
     plugin: autotools
@@ -215,6 +208,7 @@ parts:
       - libxml2-dev
       - libxslt1-dev
     stage-packages:
+      - conntrack
       - coreutils
       - qemu-utils
     install: |