Description of problem:
======================
Currently detach-tier command directly detaches the hot tier(infact the tier which is attached later).
But there may be scenarios where the customer wants to detach cold-tier.
Say for example they want to replace the old disks in cold tier.



Version-Release number of selected component (if applicable):
============================================================
3.7 upstream nightlies build http://download.gluster.org/pub/gluster/glusterfs/nightly/glusterfs/epel-6-x86_64/glusterfs-3.7dev-0.803.gitf64666f.autobuild/

glusterfs 3.7dev built on Mar 26 2015 01:04:24


How reproducible:
===============
easily

Steps to Reproduce:
==================
1.create a gluster volume of type distribute
2.attach a tier to the volume using attach-tier. 
3. Now check the volume type. It would show as a tiered volume.
4. Now issue a detach-tier will just detach the tier previously attached.
 User can't choose if he/she wants to attach or detach a cold/hot tier



Expected results:
================
Leave it to the user to choose if they want to detach cold or hot tier.