Add identity v3 project tags client

This PS adds ``project_tags_client`` to the identity v3
library. This feature enables the possibility of invoking
the following API actions:

    * update_project_tag
    * list_project_tags
    * update_all_project_tags
    * check_project_tag_existence
    * delete_project_tag
    * delete_all_project_tags

Change-Id: Iad6b3a88639bb4a0dc3aea5af2ba0162dfa19f96
Depends-On: Iec6b34c10ea1bd7103720c773b48ce130643115d
7 files changed