[APPC] Use common secret tamplate for mariadb-galera credentials

[APPC] Use common secret tamplate for mariadb-galera credentials

Use init container construction & common secret template to remove
hardcoded mysql db credentials.

For now credentials are still hardcoded in the values file to avoid
making too many changes at once.

I also had to make properties files writeable because appc uses some
weird design and modifies them inside main container while starting
odl.

Issue-ID: OOM-2275
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Change-Id: Ie36522fc1fdf92a382d5f25a54d4cb150b57ea35