More specific lxml versions in tools/pip-requires

Changed lxml version in tools/pip-requires from >=2.3,<3 to
>=2.3,<2.3.5.  pip was apparently accepting 3.0alpha as a <3 requirement
but the ordering of elements changes in that version.  Some of the
libvirt tests would pass with lxml 3.0alpha but fail with any 2.3.x
version.  The tests now pass with any version of lxml within the range
specified.

Change-Id: I2d84d32f1bf6fba2c6dbe05799a284a44d4c39d8

nova/tests/test_libvirt_config.py

@@ -366,7 +366,7 @@ class LibvirtConfigGuestDiskTest(LibvirtConfigBaseTest):
         self.assertXmlEqual(xml, """
             <disk type="network" device="disk">
               <driver name="qemu" type="qcow2"/>
-              <source name="foo.bar.com" protocol="iscsi"/>
+              <source protocol="iscsi" name="foo.bar.com"/>
               <target bus="ide" dev="/dev/hda"/>
             </disk>""")
 
@@ -387,7 +387,7 @@ class LibvirtConfigGuestDiskTest(LibvirtConfigBaseTest):
         self.assertXmlEqual(xml, """
             <disk type="network" device="disk">
               <driver name="qemu" type="raw"/>
-              <source name="pool/image" protocol="rbd"/>
+              <source protocol="rbd" name="pool/image"/>
               <auth username="foo">
                 <secret type="ceph"
                 uuid="b38a3f43-4be2-4046-897f-b67c2f5e0147"/>
@@ -530,7 +530,7 @@ class LibvirtConfigGuestInterfaceTest(LibvirtConfigBaseTest):
             <interface type="direct">
               <mac address="DE:AD:BE:EF:CA:FE"/>
               <model type="virtio"/>
-              <source dev="eth0" mode="private"/>
+              <source mode="private" dev="eth0"/>
               <virtualport type="802.1Qbh"/>
             </interface>""")