New plugin: minimize_size

It binds folders from the host machine to temporary and cache folders in the image.
This way dynamic volumes will not grow in size when running `apt-get update` etc.

plugins/minimize_size/__init__.py

@@ -0,0 +1,16 @@
+import tasks
+
+
+def resolve_tasks(tasklist, manifest):
+	tasklist.add(tasks.AddFolderMounts,
+	             tasks.RemoveFolderMounts)
+
+
+def resolve_rollback_tasks(tasklist, tasks_completed, manifest):
+	completed = [type(task) for task in tasks_completed]
+
+	def counter_task(task, counter):
+		if task in completed and counter not in completed:
+			tasklist.add(counter)
+
+	counter_task(tasks.AddFolderMounts, tasks.RemoveFolderMounts)

plugins/minimize_size/tasks.py

@@ -0,0 +1,41 @@
+from base import Task
+from common import phases
+from common.tasks import bootstrap
+from common.tasks import apt
+import os
+
+folders = ['tmp', 'var/lib/apt/lists']
+
+
+class AddFolderMounts(Task):
+	description = 'Mounting folders for writing temporary and cache data'
+	phase = phases.os_installation
+	predecessors = [bootstrap.Bootstrap]
+
+	def run(self, info):
+		info.minimize_size_folder = os.path.join(info.workspace, 'minimize_size')
+		os.mkdir(info.minimize_size_folder)
+		for folder in folders:
+			temp_path = os.path.join(info.minimize_size_folder, folder.replace('/', '_'))
+			os.mkdir(temp_path)
+
+			full_path = os.path.join(info.root, folder)
+			info.volume.partition_map.root.add_mount(temp_path, full_path, ['--bind'])
+
+
+class RemoveFolderMounts(Task):
+	description = 'Removing folder mounts for temporary and cache data'
+	phase = phases.system_cleaning
+	successors = [apt.AptClean]
+
+	def run(self, info):
+		import shutil
+		for folder in folders:
+			temp_path = os.path.join(info.minimize_size_folder, folder.replace('/', '_'))
+			full_path = os.path.join(info.root, folder)
+
+			info.volume.partition_map.root.remove_mount(full_path)
+			shutil.rmtree(temp_path)
+
+		os.rmdir(info.minimize_size_folder)
+		del info.minimize_size_folder