[DOCS] Added release-name as a watermark to Docs.

Added a water mark to galera role documentation
to make clear to deployers which release they're
working with.

Modified conf.py that adds watermark to the documentation according
to particular branch. If the current branch is master than
watermark is 'Pre-release' and if the current branch is
stable/<release-name> than watermark is <release-name>.

This is a combined implementation based on the following
patches:
- https://review.openstack.org/372565
- https://review.openstack.org/382637
- https://review.openstack.org/384140

Related-Bug: #1624489
Change-Id: Idf26db677a71bc9b3e1d22b4ca86bb7d2e5cd232

doc/source/conf.py

@@ -23,6 +23,7 @@
 # serve to show the default.
 
 import pbr.version
+import os
 
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
@@ -39,7 +40,8 @@ import pbr.version
 # ones.
 extensions = [
     'sphinx.ext.autodoc',
-    'oslosphinx'
+    'oslosphinx',
+    'sphinxmark'
 ]
 
 # Add any paths that contain templates here, relative to this directory.
@@ -303,3 +305,15 @@ texinfo_documents = [
 
 # If true, do not generate a @detailmenu in the "Top" node's menu.
 # texinfo_no_detailmenu = False
+
+watermark = os.popen("git branch --contains $(git rev-parse HEAD) | awk -F/ '/stable/ {print $2}'").read().strip(' \n\t').capitalize()
+if watermark == "":
+  watermark = "Pre-release"
+
+# -- Options for sphinxmark -----------------------------------------------
+sphinxmark_enable = True
+sphinxmark_div = 'docs-body'
+sphinxmark_image = 'text'
+sphinxmark_text = watermark
+sphinxmark_text_color = (128, 128, 128)
+sphinxmark_text_size = 70

test-requirements.txt

@@ -13,3 +13,4 @@ sphinx!=1.3b1,<1.4,>=1.2.1 # BSD
 oslosphinx>=4.7.0 # Apache-2.0
 doc8 # Apache-2.0
 reno>=1.8.0 # Apache2
+sphinxmark>=0.1.14  # Apache-2.0