Make nova-manage db purge take --all-cells

This makes purge iterate over all cells if requested. This also makes our
post_test_hook.sh use the --all-cells variant with just the base config
file.

Related to blueprint purge-db

Change-Id: I7eb5ed05224838cdba18e96724162cc930f4422e

gate/post_test_hook.sh

@@ -19,7 +19,7 @@ function archive_deleted_rows {
 }
 
 function purge_db {
-    $MANAGE $* db purge --all --verbose
+    $MANAGE db purge --all --verbose --all-cells
     RET=$?
     if [[ $RET -eq 0 ]]; then
         echo Purge successful
@@ -40,7 +40,7 @@ cell_conf=$(conductor_conf 1)
 conf="--config-file $NOVA_CONF --config-file $cell_conf"
 
 archive_deleted_rows $conf
-purge_db $conf
+purge_db
 
 set -e
 # We need to get the admin credentials to run the OSC CLIs for Placement.