Fallback if Git repository is absent

Documentation build fails during packaging if Git repository is absent.
We do not package .git directory and that is why it leads to fails during
documentation build.
With this change we are certain that it will not fail.

This change was originally proposed by Davanum Srinivas (dims):
https://review.openstack.org/287448/

Change-Id: I49dce2537ea26c168af9a67d398930042702762c

doc/source/conf.py

@@ -150,9 +150,13 @@ modindex_common_prefix = ['keystoneclient.']
 # If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
 # using the given strftime format.
 git_cmd = ["git", "log", "--pretty=format:'%ad, commit %h'", "--date=local",
-   "-n1"]
-html_last_updated_fmt = subprocess.Popen(git_cmd,
+           "-n1"]
+try:
+    html_last_updated_fmt = subprocess.Popen(git_cmd,
                                      stdout=subprocess.PIPE).communicate()[0]
+except Exception:
+    warnings.warn('Cannot get last updated time from git repository. '
+                  'Not setting "html_last_updated_fmt".')
 
 # If true, SmartyPants will be used to convert quotes and dashes to
 # typographically correct entities.