Bug #694880: nova-compute now depends upon Cheetah even when not using libvirt

Only import Cheetah when needed, as we do already with libvirt and libxml2.  This ensures that users of other virt backends don't need Cheetah to run nova-compute.

Resubmitted with pep8 violations fixed.

nova/tests/test_virt.py

@@ -33,6 +33,7 @@ flags.DECLARE('instances_path', 'nova.compute.manager')
 class LibvirtConnTestCase(test.TestCase):
     def setUp(self):
         super(LibvirtConnTestCase, self).setUp()
+        libvirt_conn._late_load_cheetah()
         self.flags(fake_call=True)
         self.manager = manager.AuthManager()
         self.user = self.manager.create_user('fake', 'fake', 'fake',

nova/virt/libvirt_conn.py

@@ -58,10 +58,9 @@ from nova.compute import instance_types
 from nova.compute import power_state
 from nova.virt import images
 
-from Cheetah.Template import Template
-
 libvirt = None
 libxml2 = None
+Template = None
 
 
 FLAGS = flags.FLAGS
@@ -88,15 +87,26 @@ flags.DEFINE_bool('allow_project_net_traffic',
 def get_connection(read_only):
     # These are loaded late so that there's no need to install these
     # libraries when not using libvirt.
+    # Cheetah is separate because the unit tests want to load Cheetah,
+    # but not libvirt.
     global libvirt
     global libxml2
     if libvirt is None:
         libvirt = __import__('libvirt')
     if libxml2 is None:
         libxml2 = __import__('libxml2')
+    _late_load_cheetah()
     return LibvirtConnection(read_only)
 
 
+def _late_load_cheetah():
+    global Template
+    if Template is None:
+        t = __import__('Cheetah.Template', globals(), locals(), ['Template'],
+                       -1)
+        Template = t.Template
+
+
 def _get_net_and_mask(cidr):
     net = IPy.IP(cidr)
     return str(net.net()), str(net.netmask())