fn: size restricted tmpfs /tmp and read-only / support (#1012)

* fn: size restricted tmpfs /tmp and read-only / support

*) read-only Root Fs Support
*) removed CPUShares from docker API. This was unused.
*) docker.Prepare() refactoring
*) added docker.configureTmpFs() for size limited tmpfs on /tmp
*) tmpfs size support in routes and resource tracker
*) fix fn-test-utils to handle sparse files better in create file

* test typo fix

api/datastore/sql/migrations/14_add_tmpfs_size_route.go

@@ -0,0 +1,27 @@
+package migrations
+
+import (
+	"context"
+
+	"github.com/fnproject/fn/api/datastore/sql/migratex"
+	"github.com/jmoiron/sqlx"
+)
+
+func up14(ctx context.Context, tx *sqlx.Tx) error {
+	_, err := tx.ExecContext(ctx, "ALTER TABLE routes ADD tmpfs_size int;")
+
+	return err
+}
+
+func down14(ctx context.Context, tx *sqlx.Tx) error {
+	_, err := tx.ExecContext(ctx, "ALTER TABLE routes DROP COLUMN tmpfs_size;")
+	return err
+}
+
+func init() {
+	Migrations = append(Migrations, &migratex.MigFields{
+		VersionFunc: vfunc(14),
+		UpFunc:      up14,
+		DownFunc:    down14,
+	})
+}