Add doc8 rule and check doc/source files

doc8 is a linter for documents and used in openstack-manuals.
It is better to enforce document linters for simple checking.
This change is to add doc8 in tox file and fix line too long
in some files.

The current rules are as bellow:
- invalid rst format - D000
- lines should not be longer than 79 characters - D001
  - RST exception: line with no whitespace except in the beginning
  - RST exception: lines with http or https urls
  - RST exception: literal blocks
  - RST exception: rst target directives
- no trailing whitespace - D002
- no tabulation for indentation - D003
- no carriage returns (use unix newlines) - D004
- no newline at end of file - D005

Change-Id: I01b11619b42eebf13cb17e1b4a2e8464a8ccc797

1 files changed, 2 insertions, 2 deletions

diff --git a/doc/source/middlewarearchitecture.rst b/doc/source/middlewarearchitecture.rst
index bf85661..9e13b97 100644
--- a/doc/source/middlewarearchitecture.rst
+++ b/doc/source/middlewarearchitecture.rst
@@ -203,8 +203,8 @@ they've been stored in the cache may continue to work. Deployments using
 `memcached`_ may use the following keystonemiddleware configuration options
 instead of an in-memory cache.
 
-* ``memcached_servers``: (optional) if defined, the memcached server(s) to use for
-  caching. It will be ignored if Swift MemcacheRing is used instead.
+* ``memcached_servers``: (optional) if defined, the memcached server(s) to use
+  for caching. It will be ignored if Swift MemcacheRing is used instead.
 * ``token_cache_time``: (optional, default 300 seconds) Set to -1 to disable
   caching completely.