Add help infomation for policy abbreviation
Add help infomation for policy abbreviation

When we create policy with long abbreviation that more than
5 chars, like: "test_abbr", API exception is raised from
Congress server side. Add some help infomation to remind users
inputing "abbreviation" arguments with right format.

Change-Id: I48cab945286aacc12c1474c5074f6febb941e723
Related-Bug: #1497881

openstack/python-congressclient