wrong centos box version in vagrant/centos7-build/Vagrantfile

[cokabug]

The box 'boxcutter/centos71' could not be found or
could not be accessed in the remote catalog. If this is a private
box on HashiCorp's Atlas, please verify you're logged in via
vagrant login. Also, please double-check the name. The expanded
URL and error message are shown below:

URL: ["https://atlas.hashicorp.com/boxcutter/centos71"]
Error: The requested URL returned error: 404 Not Found

[cokabug]

the box name should be boxcutter/centos72 now

[b-ranto]

Hey, we are trying to move away from the vagrant builds towards more regular builds that use mock as defined in these two jenkins scripts:

https://github.com/ceph/ceph-build/blob/master/calamari-setup/build/build
https://github.com/ceph/ceph-build/blob/master/calamari-build/build/build_rpm

However, feel free to create an upstream PR for 1.3 with the change so that we could review.