Starting in focal, the ubuntu version of corosync package synced in from
debian includes node1 as the default name for the local node with a nodeid
of 1. This causes the cluster to have knowledge of this extra node1 node,
which affects quorum, etc. Installing the charm's corosync.conf file
before package installation prevents this conditioning from happening.
Additionally this change removes some Xenial bits in the charm and always
includes a nodelist in corosync.conf as it is compulsory in focal and
newer. It is optional in the bionic packages, so we'll always just
render the nodelist.
Change-Id: I06b9c23eb57274f0c99a3a05979c0cabf87c8118
Closes-Bug: #1874719
d1191dbcab