Add OpenStack logo in the footer

- add small transparent OpenStack logo in the slide footer
- logo and footer design is from OpenStack's
  OpenStackPowered-April2015.ppt master slides
- white background changed to transparent
- logo is pushed back with z-index: -2 (so it is behind background pic
  in the title slide)

Change-Id: Iccdd9a968816f00dde60230df45c5a21d212d38e

doc/upstream-training/source/_static/custom.css

@@ -40,3 +40,12 @@ article h1 {
 article h2 {
     color: rgb(133,11,19);
 }
+
+.slide-footer {
+    position: absolute;
+    bottom: 1ex;
+    right: 1em;
+    font-size: 66%;
+    text-align: right;
+    z-index: -2;
+}

doc/upstream-training/source/conf.py

@@ -274,6 +274,7 @@ extensions += [
 #slide_theme = 'slides'
 slide_theme = 'single-level'
 slide_levels = 3
+slide_footer = '<img alt="OS logo" src="_static/os_logo_small.png" style="width: 4em;">'
 
 # Place custom static assets in the _static directory and uncomment
 # the following lines to include them