Use uuid.uuid4 to generate model name

Take the last 12 characters from a uuid for generating the model name.
uuid.uuid4 takes into account the host and the current timestamp and
should be sufficient for our needs.

unit_tests/test_zaza_charm_lifecycle_func_test_runner.py

@@ -7,8 +7,10 @@ import unit_tests.utils as ut_utils
 class TestCharmLifecycleFuncTestRunner(ut_utils.BaseTestCase):
 
     def test_generate_model_name(self):
-        mname = lc_func_test_runner.generate_model_name()
-        self.assertTrue(mname.startswith('zaza-'))
+        self.patch_object(lc_func_test_runner.uuid, "uuid4")
+        self.uuid4.return_value = "longer-than-12characters"
+        self.assertEqual(lc_func_test_runner.generate_model_name(),
+                         "zaza-12characters")
 
     def test_parser(self):
         # Test defaults

zaza/charm_lifecycle/func_test_runner.py

@@ -1,9 +1,9 @@
 import argparse
 import asyncio
-import datetime
 import logging
 import os
 import sys
+import uuid
 
 import zaza.charm_lifecycle.configure as configure
 import zaza.charm_lifecycle.destroy as destroy
@@ -14,8 +14,7 @@ import zaza.charm_lifecycle.test as test
 
 
 def generate_model_name():
-    timestamp = datetime.datetime.now().strftime('%Y%m%d%H%M%S')
-    return 'zaza-{}'.format(timestamp)
+    return 'zaza-{}'.format(str(uuid.uuid4())[-12:])
 
 
 def func_test_runner(keep_model=False, smoke=False):