[DOC] Add missing graphviz module

Tox fails due to missing graphviz module.
Added the installation in tox.ini files
(Ticket: IT-26010)

Issue-ID: OOM-2342

Change-Id: Ica9f750c6644386d2dc3fac6a786463929c320d6
Signed-off-by: Andreas Geissler <andreas-geissler@telekom.de>
diff --git a/docs/tox.ini b/docs/tox.ini
index 8bec1b5..94bc3ff 100644
--- a/docs/tox.ini
+++ b/docs/tox.ini
@@ -9,7 +9,9 @@
     -r{toxinidir}/requirements-docs.txt
     -chttps://raw.githubusercontent.com/openstack/requirements/stable/yoga/upper-constraints.txt
     -chttps://git.onap.org/doc/plain/etc/upper-constraints.onap.txt?h=master
+allowlist_externals = sudo
 commands =
+    sudo apt install graphviz
     sphinx-build -W -q -b html -n -d {envtmpdir}/doctrees {toxinidir} {toxinidir}/_build/html
 
 [testenv:docs-linkcheck]
@@ -18,7 +20,9 @@
     -r{toxinidir}/requirements-docs.txt
     -chttps://raw.githubusercontent.com/openstack/requirements/stable/yoga/upper-constraints.txt
     -chttps://git.onap.org/doc/plain/etc/upper-constraints.onap.txt?h=master
+allowlist_externals = sudo
 commands =
+    sudo apt install graphviz
     sphinx-build -W -q -b linkcheck -d {envtmpdir}/doctrees {toxinidir} {toxinidir}/_build/linkcheck
 
 [testenv:docs-spellcheck]
@@ -27,6 +31,8 @@
     -r{toxinidir}/requirements-docs.txt
     -chttps://raw.githubusercontent.com/openstack/requirements/stable/yoga/upper-constraints.txt
     -chttps://git.onap.org/doc/plain/etc/upper-constraints.onap.txt?h=master
+allowlist_externals = sudo
 commands =
+    sudo apt install graphviz
     sphinx-build -b spelling -d {envtmpdir}/doctrees {toxinidir} {toxinidir}/_build/spellcheck