empty `MYSQL_USERNAME` is not possible

[Cougar]

percona-clustercheck/clustercheck

Line 38 in d5da8be

MYSQL_USERNAME="${MYSQL_USERNAME:=-clustercheckuser}"

always sets MYSQL_USERNAME to "clustercheckuser" if it is missing or empty. This will add --user parameter here

percona-clustercheck/clustercheck

Lines 49 to 51 in d5da8be

	if [[ -n "$MYSQL_USERNAME" ]]; then
	EXTRA_ARGS="$EXTRA_ARGS --user=${MYSQL_USERNAME}"
	fi

and with that it always overrides username in /etc/my.cnf.

The right expansion should be:

MYSQL_USERNAME="${MYSQL_USERNAME-clustercheckuser}"

Also look at #23 (comment)