Replace unicode with six.text_type

The Unicode type is 'unicode' in Python 2 and 'str' on Python 3: replace
unicode with six.text_type to make Nova compatible with Python 2 and
Python 3.

This patch was generated by the following tool (revision e760664379c3) with the
operation "unicode":
https://bitbucket.org/haypo/misc/src/tip/python/sixer.py

Manual change:

* Replace "isinstance(value, str) or isinstance(value, unicode)"
  with "isinstance(value, six.string_types)" in nova/api/ec2/ec2utils.py

* Revert changes in strings in:

  - nova/compute/api.py
  - nova/hacking/checks.py
  - nova/tests/unit/api/openstack/test_wsgi.py
  - nova/utils.py

* Revert changes in nova/tests/unit/test_hacking.py: tests must use
  "unicode()". The nova.hacking module will probably need other changes
  to support Python 3.

* Reformat nova/tests/unit/objects/test_instance_action.py and
  nova/tests/unit/virt/hyperv/test_hypervapi.py to 80 columns

Blueprint nova-python3
Change-Id: I7ced236b6f8f8b6a5d2e7fee3c4f0ba4d72c21fb

nova/utils.py

@@ -424,7 +424,7 @@ def utf8(value):
     http://github.com/facebook/tornado/blob/master/tornado/escape.py
 
     """
-    if isinstance(value, unicode):
+    if isinstance(value, six.text_type):
         return value.encode('utf-8')
     assert isinstance(value, str)
     return value
@@ -600,7 +600,7 @@ def make_dev_path(dev, partition=None, base='/dev'):
 
 def sanitize_hostname(hostname):
     """Return a hostname which conforms to RFC-952 and RFC-1123 specs."""
-    if isinstance(hostname, unicode):
+    if isinstance(hostname, six.text_type):
         hostname = hostname.encode('latin-1', 'ignore')
 
     hostname = re.sub('[ _]', '-', hostname)
@@ -1076,7 +1076,7 @@ def get_system_metadata_from_image(image_meta, flavor=None):
     prefix_format = SM_IMAGE_PROP_PREFIX + '%s'
 
     for key, value in image_meta.get('properties', {}).iteritems():
-        new_value = safe_truncate(unicode(value), 255)
+        new_value = safe_truncate(six.text_type(value), 255)
         system_meta[prefix_format % key] = new_value
 
     for key in SM_INHERITABLE_KEYS: