Fix external secret name in mariadb-init

Fix external secret name in mariadb-init

mariadb-init chart should play nicely with mariadb-galera as it
simplifies migration to common mariadb instance.

Unfortunately after adding the support for common secret template I
didn't pay enough attention to consistent naming convention and
mariadb-galera and mariadb-init chart ended up being incompatible. To
fix that let's just rename the mariadb-init chart config option to
match exactly the one used in mariadb-galera chart.

Issue-ID: OOM-2248
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Change-Id: I621804821292e2bd0b5b1dd3f010629d1cb5471f