Assure python libselinux python bindings are installed

Avoid akward error during templating of Dockerfile which happens
on selinux enabled machines when ansible template module fails
due to missing selinux bindings.

By using sitepackages=true in tox.ini we likely inherit the libselinux
bindings from the system when we create the virtualenvs.

Workaround for ansible/ansible#34340 which
happens even if selinux is set to permissive.

Documents selinux issues in install documentation in order to inform
users about what they could encounter.

Fixes: ansible#1724
Signed-off-by: Sorin Sbarnea <ssbarnea@redhat.com>

tox.ini

@@ -27,6 +27,8 @@ deps =
     #git+https://github.com/manahl/pytest-plugins.git@54ca3c0#egg=pytest-verbose-parametrize&subdirectory=pytest-verbose-parametrize
     pytest-xdist<1.28.0  # pyup: < 1.28 # blocked by `pytest`
     shade==1.31.0
+    # https://github.com/ansible/molecule/issues/1724
+    selinux; platform_system=='Linux'
     yapf>=0.25.0,<0.27  # pyup: < 0.27 # disable updates, conflicts with flake8 as per https://github.com/ansible/molecule/pull/1915
 
     ansible25: ansible>=2.5,<2.6
@@ -55,11 +57,6 @@ commands =
     lint: python -m yamllint -s .
 whitelist_externals =
     find
-# Enabling sitepackages is needed in order to avoid encountering exceptions
-# caused by missing selinux python bindinds in ansible modules like template.
-# Selinux python bindings are binary and they cannot be installed using pip
-# in virtualenvs. Details: https://github.com/ansible/molecule/issues/1724
-sitepackages = true
 
 [testenv:lint]
 # f'' is used inside docs