Add libxml2-devel package dependency

Without installing libxm2-dev or libxml2-devel packages installed the
uwsgi services will fail to start.

This was missed by the role gate because the libxml2-dev package was
installed as a part of the keystone installation.

Change-Id: Iebfe5a0efee16d931bf8a280b5fad3da90fd22a7

vars/redhat-7.yml

@@ -25,6 +25,7 @@ nova_distro_packages:
   - iptables
   - postgresql-devel
   - which
+  - libxml2-devel
 
 # Spice console yum packages
 nova_spice_distro_packages:

vars/suse-42.yml

@@ -26,6 +26,7 @@ nova_distro_packages:
   - iptables
   - postgresql-devel
   - which
+  - libxml2-devel
 
 # Spice console zypper packages
 nova_spice_distro_packages:

vars/ubuntu-16.04.yml

@@ -25,6 +25,7 @@ nova_distro_packages:
   - git
   - libpq-dev
   - iptables
+  - libxml2-dev
 
 # Spice console apt packages
 nova_spice_distro_packages: