scripts/vagrant: do not treat SPDK_VAGRANT_DISTRO as array

Dash and double dash ("-" and "--") getopts params are safely
shifted using "shift" and "OPTIND". After that it's safe to
concatenate param array to string using "$*", especially that
there is only 1 positional parameter, which is distro version.

Previous approach ("$@" as array) made "export" impossible and
as a result VM was created with Fedora 28 distro on each run.

Change-Id: I48fa588220f8b7f46c561d480ee32920847c7903
Signed-off-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/476128
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Maciej Wawryk <maciejx.wawryk@intel.com>
Reviewed-by: Pawel Kaminski <pawelx.kaminski@intel.com>

scripts/vagrant/create_vbox.sh

@@ -130,9 +130,9 @@ done
 
 shift "$((OPTIND-1))"   # Discard the options and sentinel --
 
-SPDK_VAGRANT_DISTRO=( "$@" )
+SPDK_VAGRANT_DISTRO="$*"
 
-case "${SPDK_VAGRANT_DISTRO[0]}" in
+case "${SPDK_VAGRANT_DISTRO}" in
 	centos7)
 		export SPDK_VAGRANT_DISTRO
 	;;