Bump openstackdocstheme to 1.20.0

Some options are now automatically configured by the version 1.20:
- project
- html_last_updated_fmt
- latex_engine
- latex_elements
- version
- release.

Change-Id: I2577ca452a014402f1377474457095176d422b42

doc/requirements.txt

@@ -1,7 +1,7 @@
 # The order of packages is significant, because pip processes them in the order
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
-openstackdocstheme>=1.18.1 # Apache-2.0
+openstackdocstheme>=1.20.0 # Apache-2.0
 reno>=2.5.0 # Apache-2.0
 sphinx!=1.6.6,!=1.6.7,>=1.6.2,<2.0.0;python_version=='2.7' # BSD
 sphinx!=1.6.6,!=1.6.7,>=1.6.2;python_version>='3.4' # BSD

doc/source/conf.py

@@ -84,7 +84,6 @@ html_theme_path = [openstackdocstheme.get_html_theme_path()]
 # Output file base name for HTML help builder.
 htmlhelp_basename = '%sdoc' % project
 
-html_last_updated_fmt = '%Y-%m-%d %H:%M'
 
 # -- Options for man page output ----------------------------------------------