Add libssl-dev for Ubuntu

Rebuilding pyOpenSSL requires the SSL development libraries. This is
present in files/rpms/general, but was missed in files/apts/general.

Change-Id: Ie3c688e2d74485eb7f981363633bbddab5da7046
Merge "Add libssl-dev for Ubuntu"
Merge branch 'master' of https://github.com/openstack-dev/devstack