Use openstackdocstheme for docs and release notes

Replace the usage of oslosphinx with openstackdocstheme and follow the
porting guide[1].

[1]
https://specs.openstack.org/openstack/docs-specs/specs/pike/os-manuals-migration.html

Change-Id: I8199b787c05406ba3997fdfe26d2bfcd31b329b4
diff --git a/test-requirements.txt b/test-requirements.txt
index 64f3df7..b2051c9 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -5,7 +5,7 @@
 hacking>=0.12.0,<0.13 # Apache-2.0
 
 sphinx!=1.6.1,>=1.5.1 # BSD
-oslosphinx>=4.7.0 # Apache-2.0
+openstackdocstheme>=1.17.0  # Apache-2.0
 
 # releasenotes
 reno>=1.8.0 # Apache-2.0