Move doc8 to test requirements

doc8 is not required to use this module, only for style
checking of RST documentation.

Move from requirements.txt -> test-requirements.txt to avoid
generating a run-time dependency on doc8.

Change-Id: I3ae3909401eb53d4442e6d369228e235e1d5d60e

requirements.txt

@@ -8,7 +8,6 @@ oslo.utils>=3.33.0 # Apache-2.0
 oslo.concurrency>=3.26.0 # Apache-2.0
 stevedore>=1.20.0 # Apache-2.0
 enum34>=1.0.4;python_version=='2.7' or python_version=='2.6' or python_version=='3.3' # BSD
-doc8>=0.6.0 # Apache-2.0
 eventlet!=0.18.3,!=0.20.1,>=0.18.2 # MIT
 six>=1.10.0 # MIT
 

test-requirements.txt

@@ -4,6 +4,9 @@
 # Metrics and style
 hacking!=0.13.0,<0.14,>=0.12.0 # Apache-2.0
 
+# Documentation style
+doc8>=0.6.0 # Apache-2.0
+
 # Packaging
 mock>=2.0.0 # BSD