package: consolidate get_packages logic

Refactor get_package logic.
With this refactoring, code like
"if is_ubuntu; then install_package xxx elif is_fedora..."
can be simplified later.

Change-Id: I489bfd4cc12cc6b0b8201837f2bfb78c6881c82c
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Add cinder multi backend support to devstack

* add CINDER_MULTI_LVM_BACKEND parameter
* if CINDER_MULTI_LVM_BACKEND = True, creates 2 LVM volumes
    - Name : stack-volumes and stack-volumes2
    - Size : 5GB for each of them

Change-Id: I6e03b96e6b2aed0f0c166472300139fba9eaa68b
Merge "package: consolidate get_packages logic"
Merge "Add cinder multi backend support to devstack"