REBASED: add support for chrony on EL7

[ogajduse]

This PR is identical with #43 by @selyx but rebased to current master.
This PR should solve #71, most likely solve #22.
If you would choose a different approach, please let me know, I will try to change it.

[m8ram]

As is this change will work for RHEL6 and up but it will break for RHEL 5. Is it possible to use ntp for all ansible_distribution_major_version < '7'?

[ogajduse]

@m8ram I am happy to edit these few characters.
@geerlingguy Do you want the role to keep support for RHEL 5?

Official Red Hat's statement about RHEL 5 support:

The end of Extended Life Cycle Support is scheduled for November 30, 2020, for RHEL 5.11.

[geerlingguy]

It would be nice to still support RHEL 5, but I wouldn't hold back merging this at some point on that point. Note that I still use ntp on CentOS 7... haven't yet deployed CentOS 8 in production. Can you also add a centos8 test in .travis.yml since that should presumably work with chrony?

[ogajduse]

Tests on Centos 8 passed. Error in the Debian run is infrastructure-related.

[geerlingguy]

I merged this, but changed the way it works so you can still override ntp_package and ntp_config file. Because you can still install and use ntp on CentOS/RHEL 7 it's important to preserve the ability for people to use this role on that OS release.

For those who wish to still use ntp on CentOS 7 instead of chronyd, they can set:

ntp_package: ntp
ntp_config_file: /etc/ntp.conf

And it should work the same as before. However, since defaults are changing, I am releasing a major new version of this role.