Add ability for user to utilize external DBs for Multi-DB sample

Updated based on code review

spring-cloud-task-samples/multiple-datasources/pom.xml

@@ -52,6 +52,10 @@
 			<groupId>org.hsqldb</groupId>
 			<artifactId>hsqldb</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>org.mariadb.jdbc</groupId>
+			<artifactId>mariadb-java-client</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-test</artifactId>