Change the sudoer username

[lefreud]

Currently, all Magic Castle clusters built on MC Hub have the same centos sudoer username.

However, as mentionned by @cmd-ntrf, having a sudoer username matching the username of the cluster's owner would be more user-friendly. For instance, centos@somecluster.c3.ca would become fredfc@somecluster.c3.ca.

This change would only affect MC Hub deployments with SAML authentication enabled. Without SAML, authentication, we don't have access to the owner's username, so the sudoer username would remain centos.

When making this change, MC Hub should also show the sudoer username associated with each cluster, as mentionned in #26.