Ensure that all packaged python deps are installed on upgrade

The referenced bug indicates that if the charm upgrades on a previous
py2 version of the charm then the upgrade fails due to a missing package
(in this python3-yaml).  This patchset ensures that the python3 versions
of the dependency packages are installed.

Change-Id: I8b07de3b2f950237518c0555db1288f9b2c0aabf
Closes-Bug: #1746650

hooks/nova_compute_hooks.py

@@ -532,7 +532,7 @@ def relation_broken():
     CONFIGS.write_all()
 
 
-@hooks.hook('upgrade-charm')
+@hooks.hook('upgrade-charm.real')
 @harden()
 def upgrade_charm():
     apt_install(filter_installed_packages(determine_packages()),

hooks/upgrade-charm

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

hooks/upgrade-charm

@@ -0,0 +1,18 @@
+#!/bin/bash -e
+
+declare -a DEPS=('apt' 'netaddr' 'netifaces' 'pip' 'yaml' 'dnspython')
+
+check_and_install() {
+    pkg="${1}-${2}"
+    if ! dpkg -s ${pkg} 2>&1 > /dev/null; then
+        apt-get -y install ${pkg}
+    fi
+}
+
+PYTHON="python3"
+
+for dep in ${DEPS[@]}; do
+    check_and_install ${PYTHON} ${dep}
+done
+
+exec ./hooks/upgrade-charm.real

hooks/upgrade-charm.real

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