Allow customizing of virt-sysprep behaviour on package (#955)

Support `vagrant package` of e.g. bento-derived boxes that require SSH
hostkeys already set (as in the case of bento/debian-7 and above,) as
well as leave existing LVM UUIDs untouched (e.g. in bento/ubuntu-18.04.)

Instead of hard-coding sysprep operations, use an environment variable
to set these instead.

Use the Chef bento use case as an example for customizing sysprep
operations.

lib/vagrant-libvirt/action/package_domain.rb

@@ -38,7 +38,8 @@ module VagrantPlugins
           `qemu-img rebase -p -b "" #{@tmp_img}`
           # remove hw association with interface
           # working for centos with lvs default disks
-          `virt-sysprep --no-logfile --operations defaults,-ssh-userdir -a #{@tmp_img}`
+          operations = ENV.get('VAGRANT_LIBVIRT_VIRT_SYSPREP_OPERATIONS', 'defaults,-ssh-userdir')
+          `virt-sysprep --no-logfile --operations #{operations} -a #{@tmp_img}`
           # add any user provided file
           extra = ''
           @tmp_include = @tmp_dir + '/_include'