scripts/vagrant: unset private_key_file when password auth is used

The main purpose of this is to force vagrant into using its own
ssh keys when password authentication is requested. Normally,
when ssh.password is defined, vagrant will attempt to inject its
own ssh key, by using provided password first, and re-use the key
for the new ssh session to provision the VM.

However, some vagrant boxes may come with their own embedded
Vagrantfiles which define custom ssh keys. If password auth is
requested for such boxes, vagrant will not use its own key, nor
the ones that may be defined by the vagrant box. Instead, it will
fallback to interactive, password authentication. From the CI pool
perspective this is not desirable.

On other note, this ties to the ongoing work of building indepdent
images for the CI where vagrant boxes will be deployed with a
custom ssh key alredy provided inside the box.

Related work:
trello.com/c/gAfo9mH1/208-vagrant-improvements-box-packaging
trello.com/c/9Dxp2Y9c/248-packaging-centos78-and-freebsd1112-with-packer

Change-Id: I49035b426519d9b24bcdab573d335ee622130560
Signed-off-by: Michal Berger <michalx.berger@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/4283


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>