Install python-apt before hooks run

Previously this charm had relied on python-apt being installed by
the principle. As charms migrate to py3 this is no longer a safe
assumption and actually never was. This change ensures that
python-apt is installed before immediatly. This follows the same
pattern

Change-Id: I21ca95a9f2e410a52a7b64c99fad85749d52473a

hooks/install

@@ -1 +0,0 @@
-lxd_hooks.py

hooks/install

@@ -0,0 +1,20 @@
+#!/bin/bash
+# Wrapper to deal with newer Ubuntu versions that don't have py2 installed
+# by default.
+
+declare -a DEPS=('apt')
+
+check_and_install() {
+    pkg="${1}-${2}"
+    if ! dpkg -s ${pkg} 2>&1 > /dev/null; then
+        apt-get -y install ${pkg}
+    fi
+}
+
+PYTHON="python"
+
+for dep in ${DEPS[@]}; do
+    check_and_install ${PYTHON} ${dep}
+done
+
+exec ./hooks/install.real

hooks/install.real

@@ -0,0 +1 @@
+lxd_hooks.py

hooks/lxd_hooks.py

@@ -59,7 +59,7 @@ from charmhelpers.fetch import (
 hooks = Hooks()
 
 
-@hooks.hook()
+@hooks.hook('install.real')
 def install():
     status_set('maintenance', 'Installing LXD packages')
     if config('source'):