allow docs targets to run dot

dot is used to generate some graphics for the docs so it needs to be
allowed to be run.

Change-Id: I1d88b652d8698014da1d0e4ade95c34c1528382e
Signed-off-by: Doug Goldstein <cardoe@cardoe.com>

tox.ini

@@ -82,6 +82,7 @@ commands =
 commands = oslo_debug_helper -t ironic/tests/unit {posargs}
 
 [testenv:docs]
+allowlist_externals = dot
 # NOTE(dtantsur): documentation building process requires importing ironic
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
@@ -90,7 +91,9 @@ deps =
 commands = sphinx-build -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
-allowlist_externals = make
+allowlist_externals =
+  make
+  dot
 deps = {[testenv:docs]deps}
 commands =
   sphinx-build -b latex doc/source doc/build/pdf