Feature 8532: Added new plugin authconn tacacs

Added plugin authconn_tacacs.py Created new function validate_user() to
contain the logic for username password validation.
In authconn_tacacs, validate_user will be redefined to connect to TACACS.
Created class variables which will be collections for internal mode. For TACACS mode,
they will be roles_tacacs, projects_tacacs etc.

Change-Id: Ib7fc8900860a492a79f6d0220bcdbb582edad017
Signed-off-by: K Sai Kiran <saikiran.k@tataelxsi.co.in>