Add X-Python-Version >=2.6 to debian control file.

python-ceph complains when installed to debian squeeze about the 'with'
statement. Apparently installation tries to install the python-ceph package for
python 2.5, which does not support the 'with' statement natively.

Signed-off-by: Filippos Giannakos <philipgian@grnet.gr>
debian: require >= python2.6 for ceph as well

ceph-disk-prepare and ceph-disk-activate use the with statement,
str.format, and possibly other new features from python 2.6.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>