add env variables in place of db username and password (#50)

Author: ankita10r

src/main/resources/application.yml

@@ -18,8 +18,8 @@ spring:
 
   datasource:
     url: jdbc:mysql:thin://operations-mysql:3306/messagegateway
-    username: root
-    password: 4ET6ywqlGt
+    username: ${MYSQL_USERNAME:root}
+    password: ${MYSQL_PASSWORD:password}
     driver-class-name: org.drizzle.jdbc.DrizzleDriver
 
 # Status Callback configuration for Twilio. Port will be taken from server configuration