Update default cirros version to 0.5.2

Due to a known libvirt issue [1], fixed in 0.5.2, we're updating the
default cirros image version. Following is from the related commit:

```
Add ahci module

Without this module it isn't possible to see any attached SATA devices
once booted, even when booting from SATA based disks.

As discussed in the associated issue the OpenStack Nova project is
looking to test the `q35` machine type that requires the use of SATA
based cdrom config drives. Including and loading this module allows
these config drives to be seen correctly within launched instances.
```

[1] https://github.com/cirros-dev/cirros/issues/64

Change-Id: Ie0d7d5ad023121c834900aac47199231b4b9af05

config_tempest/constants.py

@@ -24,13 +24,13 @@ TEMPEST_WORKSPACE = os.getcwd()
 DEPLOYER_INPUT = os.path.join(os.path.expanduser("~"),
                               "tempest-deployer-input.conf")
 DEFAULT_IMAGE_DIR = 'etc'
-DEFAULT_IMAGE = ("https://download.cirros-cloud.net/0.4.0/"
-                 "cirros-0.4.0-x86_64-disk.img")
+DEFAULT_IMAGE = ("https://download.cirros-cloud.net/0.5.2/"
+                 "cirros-0.5.2-x86_64-disk.img")
 
-DEFAULT_IMAGES = [("https://download.cirros-cloud.net/0.4.0/"
-                  "cirros-0.4.0-x86_64-disk.img"),
+DEFAULT_IMAGES = [("https://download.cirros-cloud.net/0.5.2/"
+                  "cirros-0.5.2-x86_64-disk.img"),
                   ("http://images.rdoproject.org/cirros/"
-                  "cirros-0.4.0-x86_64-disk.img")]
+                  "cirros-0.5.2-x86_64-disk.img")]
 DEFAULT_IMAGE_FORMAT = 'qcow2'
 
 DEFAULT_FLAVOR_RAM = 128