Move some libvirt specific flags into a group

This is backwards compatible via deprecated groups. This change
moves the flags defined in nova/virt/libvirt/driver.py into a
"libvirt" configuration group.

There are other flags which need to be moved, but this change
is quite large, so I will handle those in follow up changes.

Now that the flags in nova/virt/libvirt/driver.py are in a libvirt
group, we can drop the libvirt_ from in front of many of the flag
names as well.

Progresses blueprint libvirt-opts-group.

DocImpact: the following flags are moved into a "libvirt" flag
group: rescue_image_id, rescue_kernel_id, rescue_ramdisk_id,
libvirt_type, libvirt_uri, libvirt_inject_password,
libvirt_inject_key, libvirt_inject_partition, use_usb_tablet,
live_migration_uri, live_migration_flag, block_migration_flag,
live_migration_bandwidth, snapshot_image_format,
libvirt_vif_driver, libvirt_volume_drivers

The following flags have had the libvirt_ dropped from the start of
their flag name now that they are in a flag group:
    libvirt_type (now driver_type)
    libvirt_uri (now conn_uri)
    libvirt_inject_password
    libvirt_inject_key
    libvirt_inject_partition
    libvirt_vif_driver
    libvirt_volume_drivers
    libvirt_disk_prefix
    libvirt_wait_soft_reboot_seconds
    libvirt_non_blocking
    libvirt_cpu_mode
    libvirt_cpu_model
    libvirt_snapshots_directory

Finally, the flag libvirt.nonblocking has been renamed to
libvirt.api_thread_pool, which is more descriptive.

Change-Id: I11e79eb5fd79c459374b9bf04707a4e6d79e993f
Move base_dir_name option to somewhere more central

The VMWare hypervisor driver is using the base_dir_name flag, but
it was defined in the libvirt driver. Given I am moving flags for
libvirt into a libvirt group, this flag needs to move somewhere
more central. It has also been renamed to the more descriptive
image_cache_subdirectory_name.

Change-Id: If016456b9f571ff0ec88621bd17e463faeef95ba
Move flags in libvirt's imagecache to the libvirt group

This change continues moving libvirt specific flags into a libvirt
specific configuration group.

Progresses blueprint libvirt-opts-group.

DocImpact: the following flags move into the libvirt group:
image_info_filename_pattern, remove_unused_base_images,
remove_unused_kernels, remove_unused_resized_minimum_age_seconds,
remove_unused_original_minimum_age_seconds, checksum_base_images,
checksum_interval_seconds.

Change-Id: Iea681c7db7efc9da0525ef8e01b4900ca71f10cf
Move flags in libvirt's imagebackend to the libvirt group

This change continues moving libvirt specific flags into a libvirt
specific configuration group.

Progresses blueprint libvirt-opts-group.

DocImpact: the following flags move into the libvirt group:
    libvirt_images_type (now images_type)
    images_volume_group
    sparse_logical_volumes
    lvm_snapshot_types
    images_rbd_pool
    images_rbd_cepf_conf

Change-Id: Ic1bb8076b139787fe5acbc0574b0c75b52bf3bd7
Move flag in libvirt's utils to the libvirt group

This change continues moving libvirt specific flags into a libvirt
specific configuration group.

Progresses blueprint libvirt-opts-group.

DocImpact: the following flag is now in the libvirt group:
    snapshot_compression (formerly libvirt_snapshot_compression)

Change-Id: I9eddec5cdb72a1313e044a3c7c6d75e8ad34533a
Move flag in libvirt's vif to the libvirt group

This change continues moving libvirt specific flags into a libvirt
specific configuration group.

Progresses blueprint libvirt-opts-group.

DocImpact: the following flag has been moved into the libvirt
group:
    use_virtio_for_bridges (formerly libvirt_use_virtio_for_bridges)

The unused flag libvirt_ovs_bridge has been removed.

Change-Id: I755139f0a470a091e36760836c9200f15aa72c37
Move flags in libvirt's volume to the libvirt group

This change continues moving libvirt specific flags into a libvirt
specific configuration group.

Progresses blueprint libvirt-opts-group.

DocImpact: the following flags have moved into the libvirt group:
    num_iscsi_scan_tries
    num_iser_scan_tries
    rbd_user
    rbd_secret_uuid
    nfs_mount_point_base
    nfs_mount_options
    num_aoe_discover_tries
    glusterfs_mount_point_base
    iscsi_use_multipath (formerly libvirt_iscsi_use_multipath)
    iser_use_multipath (formerly libvirt_iser_use_multipath)
    scality_sofs_config
    scality_sofs_mount_point
    qemu_allowed_storage_drivers

Change-Id: I78941aff4bf49fc70d91a3239deff44dbaaab6c9