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.

Progresses blueprint libvirt-opts-group.

DocImpact: the base_dir_name flag has been renamed
image_cache_subdirectory_name, although the old name will continue
to work for the forseeable future.

Change-Id: If016456b9f571ff0ec88621bd17e463faeef95ba
Merge "Move base_dir_name option to somewhere more central"