automation: use python-dev python2.7-dev in Debian and Ubuntu

automation: use python-dev python2.7-dev in Debian and Ubuntu

... instead of python2.7-dev.

We installed python2.7-dev because xen only worked with 2.7.

Installing python2.7-dev only gives python2.7-config, which causes
configure to fail because it wants python-config by default. Now xen
should work with 2.6 and above, we can install python-dev and let
distros pick the default python.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>