override file with nofile ulimit for new percona cluster

The bug report is related to 1905366 [1]. Its fix adds an override file /etc/systemd/system/mysql.service.d/charm-nofile.conf with a bigger nofile ulimit value so that mysql can accept more connections.
This is fine if percona cluster already exists and the max connection config is set to a new value.

However, if a new percona cluster is created with a max connection value, the above override file is not used because when the percona cluster is created and started for the first time, the started service is <email address hidden>, therefore the override file it uses is /etc/systemd/system/mysql@.service.d/charm-nofile.conf, which has a '@' symbol in the path.

This is how it looks like when a new percona cluster is just created.

root@juju-fb8079-ost-3:~# systemctl status mysql
● mysql.service - Percona XtraDB Cluster daemon
   Loaded: loaded (/lib/systemd/system/mysql.service; enabled; vendor preset: enabled)
  Drop-In: /etc/systemd/system/mysql.service.d
           └─charm-nofile.conf
   Active: inactive (dead) since Thu 2021-07-01 00:42:27 UTC; 10min ago
  Process: 1932 ExecStop=/etc/init.d/mysql stop (code=exited, status=0/SUCCESS)
 Main PID: 512 (code=exited, status=0/SUCCESS)

root@juju-fb8079-ost-3:~# systemctl status <email address hidden>
● <email address hidden> - Percona XtraDB Cluster daemon with config /etc/default/mysql.bootstrap
   Loaded: loaded (/lib/systemd/system/mysql@.service; disabled; vendor preset: enabled)
   Active: active (running) since Thu 2021-07-01 00:42:34 UTC; 11min ago
  Process: 2145 ExecStart=/etc/init.d/mysql start ${EXTRA_ARGS} (code=exited, status=0/SUCCESS)
  Process: 2043 ExecStartPre=/usr/share/mysql/mysql-systemd-start pre (code=exited, status=0/SUCCESS)
 Main PID: 2185 (mysqld_safe)
    Tasks: 102 (limit: 4703)
   CGroup: /<email address hidden>
           ├─2185 /bin/sh /usr/bin/mysqld_safe --wsrep-new-cluster
           └─2808 /usr/sbin/mysqld --basedir=/usr --datadir=/var/lib/percona-xtradb-cluster --plugin-dir=/usr/lib/mysql/plugin --wsrep-provider=/usr/lib/galera3/libgalera_smm.so --

Therefore the charm should create /etc/systemd/system/mysql@.service.d/charm-nofile.conf for bootstrapping process.

[1] https://bugs.launchpad.net/charm-percona-cluster/+bug/1905366