Add a flag for skipping the database setup

For example, in the run-mooc-grader container,
when the environment variable `grader_NO_DATABASE` is set to
a non-empty value, then run-mooc-grader skips the database setup
and migrations.

In create-db.sh, the -u parameter was removed from /bin/sh because
the script must not exit when the no-database flag is not set.

dbipc/rootfs/usr/local/sbin/create-db.sh

@@ -1,4 +1,12 @@
-#!/bin/sh -eu
+#!/bin/sh -e
+
+flagname="${CONTAINER_TYPE}_NO_DATABASE"
+eval flag=\$$flagname
+if [ $flag ]; then
+    echo "Skipping database setup because '${flagname}' is set."
+    exit 0
+fi
+
 
 prog=${0##*/}
 user=$1

dbipc/rootfs/usr/local/sbin/init-db.sh

@@ -1,5 +1,13 @@
 #!/bin/sh
 
+flagname="${CONTAINER_TYPE}_NO_DATABASE"
+eval flag=\$$flagname
+if [ $flag ]; then
+    echo "Skipping database setup because '${flagname}' is set."
+    exit 0
+fi
+
+
 user=$1
 db=$2
 shift 2

django/rootfs/usr/local/sbin/run-django.sh

@@ -42,7 +42,7 @@ export HOME="$app_path"
 # Use python from virtualenv if present
 [ -e "/local/venv_$app_name/bin/activate" ] && . /local/venv_$app_name/bin/activate
 
-# Ensure database state, will run manage.py migrate always
+# Ensure database state, will run manage.py migrate if databases are enabled.
 init-db.sh "$user" "$db" django-migrate.sh
 
 # With dev code, we need to rerun few init tasks