This PR fixes a regression in #7337 (0.38.0) that certbot cannot run with Apache on RHEL 6.
In RHEL 6, `distro.linux_distribution()` returns `RedHatEnterpriseServer`.
In RHEL 6:
```py
>>> import distro
>>> distro.linux_distribution()
(u'RedHatEnterpriseServer', u'6.10', u'Santiago')
>>> import platform
>>> platform.linux_distribution()
('Red Hat Enterprise Linux Server', '6.10', 'Santiago')
```
In RHEL 7:
```py
>>> import distro
>>> distro.linux_distribution()
('Red Hat Enterprise Linux Server', '7.6', 'Maipo')
>>> import platform
>>> platform.linux_distribution()
('Red Hat Enterprise Linux Server', '7.6', 'Maipo')
```
* fix to run with Apache on RHEL 6
* fix docs
