Add Pint for Horizon

Add the Pint package to global requirements. It is needed for unit
conversion support in Horizon's charts:
https://review.openstack.org/#/c/96800/

Pint project homepage: http://pint.readthedocs.org/
GitHub repo: https://github.com/hgrecco/pint

Pint is a pure python package without any external dependencies.
It is python3 compliant and has BSD license.

Partially implements: blueprint ceilometer-api-statistics-decorators-and-convertors

Change-Id: Ic9683c7a28cb9db008a974cde2daa57c9abcad61
Merge "Add Pint for Horizon"