Fixes minor errors and warning in Sphinx build

 * Fixes error : doc/source/backends/powerdns.rst:29: ERROR: Malformed table.
 * Fixes error : doc/source/configuration.rst:43: ERROR: Malformed table.
 * Fixes error : doc/source/getting-started.rst:285: ERROR: Duplicate target name, cannot be used as a unique reference: "installing designate"
 * Fixes warning : WARNING: html_static_path entry u'/opt/stack/designate/doc/source/_static' does not exist

Change-Id: I48e5c8c2c04a50c69d39b49a743a155c40350b21
Closes-Bug: #1377619