Add requirement pbr in documentation
Add requirement pbr in documentation

The requirement 'pbr' should be installed using pip.

Change-Id: Ifa67fb675deb853711c771662b5302d2c4245ab0

savinash47/openstack-doc-tools,savinash47/openstack-doc-tools,openstack/openstack-doc-tools,savinash47/openstack-doc-tools,openstack/openstack-doc-tools