Add development libraries needed by Pillow

Change I8da7dd95ae24cf06dc7bdc300fcf39947a6df093 added Pillow build deps
to nodepool thick slaves.  This means that Pillow 3 will work in unit
tests.

Make the matching change to allow Pillow 3.0.0 to work under devstack.

The longer term aim is to remove temporary upper cap.

Change-Id: I2bec8cf1bfeaaa6ae329704229fdeb86d26e55c7
diff --git a/files/debs/general b/files/debs/general
index 9b27156..1215147 100644
--- a/files/debs/general
+++ b/files/debs/general
@@ -8,6 +8,7 @@
 graphviz # needed for docs
 iputils-ping
 libffi-dev # for pyOpenSSL
+libjpeg-dev # Pillow 3.0.0
 libmysqlclient-dev  # MySQL-python
 libpq-dev  # psycopg2
 libssl-dev # for pyOpenSSL