tox testing support for client wheel packages

Add tox infrastructure to test client wheel packages workflow:

* build client packages
* install client packages
* ipa-run-tests --ipaclient-unittests under Python 2 and 3
* pylint of client packages under Python 2 and 3
* placeholder packages work as expected

Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Martin Basti <mbasti@redhat.com>

configure.ac

@@ -276,6 +276,7 @@ AC_CONFIG_COMMANDS([po/POTFILES.in],
 			-path "./${PACKAGE}-*" -prune -o dnl dist directories
 			-path '*/build' -prune -o dnl Python builds
 			-path '*/dist' -prune -o dnl Python dists
+			-path './.tox' -prune -o dnl Python tox test
 			-path './conf*' -prune -o dnl generated by configure
 			-name '*.py' -print -o dnl
 			-name '*.c' -print -o dnl