Raise a DeprecationWarning when openstack.common.deprecated is imported

We now have a deprecated package in Oslo where deprecated modules /
packages will be moved. In order to make this more clear and warn users,
this patch adds a warning in deprecated's __init__.py module.

Change-Id: Id7097da1b034499fec1b6d44a358223476d9ba83
Merge "Raise a DeprecationWarning when openstack.common.deprecated is imported"