Install "pyroute2" as a doc job depedency

By adding this library to the doc requirements file, tox will install
it limiting the upper version according to
"requirements/upper-constraints.txt".

"pyroute2" is called in tox doc job as mentioned in [1]. Since version
0.6.0, the library structure changed. To keep backwards compatibility,
all modules are imported from the main namespace. As reported in the
referred bug, the library "mitogen" must be installed first.

In stable releases that use older "pyroute2" versions, this library is
not needed but this patch should enforce the upper library version.

This patch also changes the "docs" and "pdf-docs" tox definitions.

Related-Bug: #1928913

[1]2a4074918c/doc/source/conf.py (L33-L40)

Conflicts:
    doc/requirements.txt
    tox.ini

Change-Id: I023191538679f2ee0d8dfb2f6231e848fdfbf86c
(cherry picked from neutron commit 9e6b7a2284a39ae74d2092083165e3fdf4cb8f33)

doc/requirements.txt

@@ -8,3 +8,6 @@ openstackdocstheme>=1.20.0 # Apache-2.0
 doc8>=0.6.0 # Apache-2.0
 oslotest>=3.2.0 # Apache-2.0
 reno>=2.5.0 # Apache-2.0
+# NOTE(ralonsoh): pyroute2 module should be installed and the version limited
+# according to upper-constraints.txt
+pyroute2>=0.5.3;sys_platform!='win32' # Apache-2.0 (+ dual licensed GPL2)

tox.ini

@@ -96,17 +96,14 @@ envdir = {toxworkdir}/docs
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/train}
   -r{toxinidir}/doc/requirements.txt
-commands =
-  rm -rf doc/build/html
-  doc8 doc/source devstack releasenotes/source vagrant rally-jobs
-  sphinx-build -W -b html doc/source doc/build/html
+  -r{toxinidir}/requirements.txt
+commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
 basepython = python3
 envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 commands =
-  rm -rf doc/build/pdf
   sphinx-build -W -b latex doc/source doc/build/pdf
   make -C doc/build/pdf