Move blockbox DB volume to home directory

Moved db_data directory to ~/.blockbox/db_data because it's owned by
MySQL in the container so tox will failed with it during *.pyc files
cleanup.

Change-Id: Ib2c57d57193cd94108927bfd3a4bf944941976e9

contrib/block-box/docker-compose.yml

@@ -19,7 +19,7 @@ services:
             cindernet:
                 ipv4_address: 172.49.49.6
         volumes:
-            - ./.db_data:/var/lib/mysql
+            - ~/.blockbox/db_data:/var/lib/mysql
             - ./docker-entrypoint-initdb.d:/docker-entrypoint-initdb.d
     cinder-api:
         image: debian-cinder