scripts/vagrant: fix exporting array in bash script

In create_vhost_vm we export SDPK_VAGRANT_DISTRO
variable to vagrant subprocess that is called
during the script.
But if SPDK_VAGRANT_DISTRO is an array variable,
then Vagrant will not see it
and always default to ubuntu16.
This small patch fixes that.

Change-Id: I45e9ed4f847e4fa10b90650a613f9cfe17ee38e3
Signed-off-by: Vitaliy Mysak <vitaliy.mysak@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/1676


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-by: Maciej Wawryk <maciejx.wawryk@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>