make utils.mkfs() more general
make utils.mkfs() more general

While types other than 'vfat' don't seem to be used at present,
it seems better to allow labels on all file system types,
for future maintenance, and to at least alleviate confusion.

Change-Id: I80110eff577e56e9e48354813efbc3e5488d6257

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects