fuel_agent: image build directory can be passed via command line

Make it possible to set the image build directory via the command
line. Also create the image build directory if it doesn't exist.

The default image build directory (/tmp) can be too slow/small.
In particular if fuel_agent runs in a container (which is the case
on the Fuel master node) /tmp resides in the container filesystem
which is backed by dm-snapshot. dm-snapshot is known to have a poor
write performance, especially for large(er) files like OS images.
By default docker storage is a loopback file residing in
/var/lib/docker. Thus the data written to the OS image being built
goes through a number of layers:

ext4 -> loop -> dm-snapshot (/var/lib/docker-> loop ->
     -> ext4 (/var/lib/docker) -> dm-linear (os-varlibdocker) ->
     -> hard drive

which causes substantial IO overhead. If the master node runs on a VM
rather than a physcial hardware the above chain gets even more
complicated (and write performance gets really afwul):

ext4 -> loop -> dm-snapshot (/var/lib/docker-> loop ->
     -> ext4 (/var/lib/docker) -> dm-linear (os-varlibdocker) ->
     -> virtual hard drive (qemu) -> qcow2 -> ext4 (host)

Using a host volume (mounted at /var/lib/fuel/ibp) as an image build
directory eliminates the extra layers so the written data goes through

ext4 -> loop -> ext4 -> dm-linear (os-var) -> hard drive

so the IBP image build time gets reasonable.

Related-Bug: #1457643
Change-Id: Idcafd23e659fcc7c0d7e6087501799834234b34c

fuel_agent/manager.py

@@ -67,11 +67,6 @@ opts = [
         default='empty_rule',
         help='Correct empty rule for udev daemon',
     ),
-    cfg.StrOpt(
-        'image_build_dir',
-        default='/tmp',
-        help='Directory where the image is supposed to be built',
-    ),
     cfg.StrOpt(
         'image_build_suffix',
         default='.fuel-agent-image',
@@ -85,6 +80,11 @@ cli_opts = [
         default='nailgun',
         help='Data driver'
     ),
+    cfg.StrOpt(
+        'image_build_dir',
+        default='/tmp',
+        help='Directory where the image is supposed to be built',
+    ),
 ]
 
 CONF = cfg.CONF
@@ -508,6 +508,7 @@ class Manager(object):
                   'Starting build process.')
         try:
             LOG.debug('Creating temporary chroot directory')
+            utils.makedirs_if_not_exists(CONF.image_build_dir)
             chroot = tempfile.mkdtemp(
                 dir=CONF.image_build_dir, suffix=CONF.image_build_suffix)
             LOG.debug('Temporary chroot: %s', chroot)

fuel_agent/tests/test_manager.py

@@ -445,8 +445,10 @@ class TestManager(test_base.BaseTestCase):
                       chroot='/tmp/imgdir',
                       uri='http://fakemos')],
             mock_bu.add_apt_preference.call_args_list)
-        mock_utils.makedirs_if_not_exists.assert_called_once_with(
-            '/tmp/imgdir/proc')
+        self.assertEqual([
+            mock.call('/tmp'),
+            mock.call('/tmp/imgdir/proc')],
+            mock_utils.makedirs_if_not_exists.call_args_list)
         self.assertEqual([
             mock.call('tune2fs', '-O', '^has_journal', '/dev/loop0'),
             mock.call('tune2fs', '-O', 'has_journal', '/dev/loop0')],