ansible: run apt-get update before install packages

Run apt-get update before installing PXEmngr dependencies, otherwise,
Ansible fails to run the installation:
failed: [chroot] =>
(item=apache2,libapache2-mod-wsgi,python-ipaddr,python-pip,git) =>
{"failed": true, "item":
"apache2,libapache2-mod-wsgi,python-ipaddr,python-pip,git"}
stderr: E: There are problems and -y was used without --force-yes

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>