Make '_short_id' method a public method
Make '_short_id' method a public method

There are many exactly the same '_short_id' methods in v1/shell,
which are used as inner methods. This patch sets '_short_id' to be
a public method, it's also easy for test.

Change-Id: Id530ea01ca9815ef9a4aa95274fd303c25c698d3
Closes-bug: #1520014

stackforge/python-senlinclient,openstack/python-senlinclient,openstack/python-senlinclient,stackforge/python-senlinclient