Add python-dev to general requirements

Package python-openstackclient depends on pycrypto, and pycrypto depends
on python-dev. Since python-openstackclient should be installed in any
case, python-dev should be in general requirements.

Change-Id: I2d6f5919bdc1175252c78cd203a8769d3892b923
Closes: bug 1292477
Merge "Add python-dev to general requirements"