Fixes for CentOS 8 (#5213)

* Fix python3-libselinux installation for RHEL/CentOS 8

In bootstrap-centos.yml we haven't gathered the facts,
so #5127 couldn't work

Minimum ansible version to run kubespray is 2.7.8,
so ansible_distribution_major_version is defined an there is no need to default it

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>

* Restart NetworkManager for RHEL/CentOS 8

network.service doesn't exist anymore
 # systemctl status network
 Unit network.service could not be found.

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>

* Add module_hotfixes=True to docker / containerd yum repo config

https://bugzilla.redhat.com/show_bug.cgi?id=1734081
https://bugzilla.redhat.com/show_bug.cgi?id=1756473


Without this setting you end up with the following error:
 # yum install docker-ce
 Failed to set locale, defaulting to C
 Last metadata expiration check: 0:03:21 ago on Thu Sep 26 22:00:05 2019.
 Error:
  Problem: package docker-ce-3:19.03.2-3.el7.x86_64 requires containerd.io >= 1.2.2-3, but none of the providers can be installed
   - cannot install the best candidate for the job
   - package containerd.io-1.2.2-3.3.el7.x86_64 is excluded
   - package containerd.io-1.2.2-3.el7.x86_64 is excluded
   - package containerd.io-1.2.4-3.1.el7.x86_64 is excluded
   - package containerd.io-1.2.5-3.1.el7.x86_64 is excluded
   - package containerd.io-1.2.6-3.3.el7.x86_64 is excluded
 (try to add '--skip-broken' to skip uninstallable packages or '--nobest' to use not only best candidate packages)

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>