Manipulating Linux /etc/group group members (Ref: Issue# 11102)

(1) modules/groupadd.py : Extending by adding three new functions.
adduser, deluser, members . For now it assumes an OS command ‘gpasswd’
is available
.

(2) states/group.py : states.group.present state function is refactored
(along the lines of states.users.present)  to support the following new
options.
  - addusers
  - delusers
  - members

This patch is for review now. This  is currently tested on CentOS(RHEL)
distributions.
Old group.present() state function is renamed as present_old() for now.