Adding the timesync_ntp_allow variable for setuping a server as source server

[florian0410]

We started to use the role in a private cloud and we are using a single server that connect to externals ntp servers for synchronization while other are offline.

We would like to use it as an NTP server and client at the same time.
From what I read we need to use the configuration allow https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/system_administrators_guide/sect-understanding_chrony_and-its_configuration

We should add a new variable to handle this in timesync/templates/chrony.conf.j2, for example:

timesync_allow_network:
  - ip_net: 127.0.1.0/24

Which would result in the vars like this:

{% if timesync_allow_network is defined %}
{% for value in timesync_allow_network %}
{{ 'allow' if 'ip_net' in value and value['ip_net'] }} {{value['ip_net'] }}
{% endfor %}
{% endif %}

Resulting in in the configuration as follow :

allow 127.0.1.0/24

[mlichvar]

As currently documented, this role is meant only for configuring NTP clients and/or PTP slaves. It might make sense to extend it into a NTP server, but we need to consider all supported use cases and how it will work with future improvements. For instance, if the role is configured with a single PTP domain, there is no ntpd/chronyd currently running. Should it be started? Or would it be better to rely on a separate "NTP server" role?

PR #46 is a similar issue.