lvm pv management

[dwlehman]

For LVM, we need to be able to manage PVs in several different ways:

1. define set of disks on which to create PVs
2. define set of existing block devices to set as PVs for the VG
3. define set of existing block devices to set as PVs for a specific LV

Users will eventually need to be able to define a set of PVs for a specific LV without implicitly
also modifying the VG's PV set.

My current thinking is that #2 and #3 need to be controlled through two distinct variables.
PVs for the VG could either be lvm_pvs or lvm_vg_pvs, while PVs for a specific LV could
be lvm_lv_pvs.

[dwlehman]

An empty disk list and empty pv lists w/ an existing vg means leave the pv set as it is.