Add dict wrapper

Dict wrapper is used for string format method using
postponed keys resolving.
With this class, format will not fail in case if key not found,
and leaves the unknown key in the string 'as is'.
This behaviour is required as a workaround for strings with
bash-like variables like ${SOMEVAR} to not fail the format because
of 'unknown key: SOMEVAR'.
Returning the 'format' instead of 'replace' is required to use
the escaping of the '{{' '}}' in the string to get '{' '}'.

Change-Id: I1c2bbb083a544eb366c7b00eb1002cc4da652c33

devops/helpers/cloud_image_settings.py

@@ -12,10 +12,11 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from copy import deepcopy
 import os
 
+from devops.helpers.helpers import format_data
 from devops.helpers import subprocess_runner
+
 from devops import logger
 
 
@@ -78,21 +79,17 @@ def generate_cloud_image_settings(cloud_image_settings_path, meta_data_path,
                              " - sudo route add default gw "
                              "{gateway} {interface_name}")
 
-    # FIXME this hack should be rewrited!
-    # Workaround to be able pass bash-style variables ${}
-    fmt_user_data_content = deepcopy(user_data_content)
-    for _key in data_context.keys():
-        _repl = "{{{0}}}".format(_key)
-        fmt_user_data_content = \
-            fmt_user_data_content.replace(_repl, data_context[_key])
+    fmt_user_data = format_data(user_data_content, data_context)
+    logger.debug("user_data contains next data: \n{}".format(fmt_user_data))
+
     with open(user_data_path, 'w') as f:
-        f.write(fmt_user_data_content)
-    logger.debug(
-        "user_data contains next data:\n{}".format(fmt_user_data_content))
+        f.write(fmt_user_data)
+
     # Generate cloud_ISO
     cmd = "genisoimage -output {} " \
           "-volid cidata -joliet " \
           "-rock {} {}".format(cloud_image_settings_path,
                                user_data_path,
                                meta_data_path)
+
     subprocess_runner.Subprocess.check_call(cmd)

devops/helpers/helpers.py

@@ -18,6 +18,7 @@ import functools
 import os
 import signal
 import socket
+import string
 import time
 import warnings
 # noinspection PyPep8Naming
@@ -424,3 +425,21 @@ def utc_to_local(t):
     t = t.replace(tzinfo=tz.tzutc())
     # convert to local timezone
     return t.astimezone(tz.tzlocal())
+
+
+def format_data(data_content, data_context):
+    """Dict wrapper.
+
+    Dict wrapper that returns key name
+    in case of key missing in the dictionary
+    """
+
+    class temp_dict(dict):
+        def __init__(self, kw):
+            self.__dict = kw
+
+        def __getitem__(self, key):
+            return self.__dict.get(key, '{' + str(key) + '}')
+
+    return string.Formatter().vformat(data_content, [], temp_dict(
+        data_context))