add ability to create VMs with disks stored in LVM

Rationale
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`QEmu` allows several types of storages. For local storage it is possible to use regular block device, `raw` image file
or special format called `qcow2`. OpenStack offers only `raw` or `qcow2` disks. Historically our (Grid Dynamics)
infrastructure was based on `qemu-kvm` with `LVM` as disk storage. When we started moving it to OpenStack based
infrastructure we measured performance of `qcow2` disks.

In brief, our tests show performance degradation on `qcow2` disks comparing to `LVM` is 5-50% depending on operation.
The biggest difference (50%) was archived in sequential IO of very big data.

Proposal
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Add ability for administrator to choose, which type of local storage he wants to use: `raw`, `LVM` or `qcow2`.

We propose to introduce a new flag `libvirt_images_type` with possible values: `raw`, `lvm`, `qcow2`, `default`.

Boolean flag `use_cow_images` should be ignored, if `libvirt_images_type` is set to one of possible values,
but keep working, if `libvirt_images_type` flag is not set by user.
New flag `libvirt_images_volume_group` should be introduced for specifying volume group name for disks storage.
Boolean option `libvirt_sparse_logical_volumes` should select which type
of logical volumes will be created (sparsed with virtualsize or usual logical volumes with full space allocation). Default value for this option is `False`.