Bump the openstackdocstheme extension to 1.20

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

Change-Id: I22c45438c1f94199826ca314f2148f3fac2678f4

doc/requirements.txt

@@ -2,4 +2,4 @@
 # push that content due to some global check.
 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
-openstackdocstheme>=1.18.1 # Apache-2.0
+openstackdocstheme>=1.20.0 # Apache-2.0

releasenotes/source/conf.py

@@ -55,7 +55,6 @@ source_suffix = '.rst'
 master_doc = 'index'
 
 # General information about the project.
-project = u'paunch Release Notes'
 copyright = u'2016, OpenStack Foundation'
 
 # The version info for the project you're documenting, acts as replacement for
@@ -199,17 +198,6 @@ htmlhelp_basename = 'GlanceReleaseNotesdoc'
 
 # -- Options for LaTeX output ---------------------------------------------
 
-latex_elements = {
-    # The paper size ('letterpaper' or 'a4paper').
-    # 'papersize': 'letterpaper',
-
-    # The font size ('10pt', '11pt' or '12pt').
-    # 'pointsize': '10pt',
-
-    # Additional stuff for the LaTeX preamble.
-    # 'preamble': '',
-}
-
 # Grouping the document tree into LaTeX files. List of tuples
 # (source start file, target name, title,
 #  author, documentclass [howto, manual, or own class]).