version: add "v" to version string

For example, VERSION_STRING will now be "SPDK v17.10-pre" instead of the
current "SPDK 17.10-pre".

This matches the release tags that we use, and it will generally be more
obvious that it's a version.

Change-Id: I0770124991b18e328f3c47f22e88798950aa8b26
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-on: https://review.gerrithub.io/376669


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: John Kariuki <John.K.Kariuki@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>