Initial rewrite to consolidate VM image build scripts into one.
There may be some very sharp edges here while refactoring.

 - Move amd64/mk-vmimage.sh -> scripts/mk-vmimage.sh.
 - Remove vm-base target from Makefile.vm.
 - In vm-image target, use getopts flags for argument passing.
 - Create tools/vmimage.subr, containing default and prototype
   for the following functions that are used to drive the build,
   run in this order:

   vm_install_base()
   vm_extra_install_base()
   vm_extra_install_packages()
   vm_extra_install_ports()
   vm_extra_enable_services()
   vm_extra_pre_umount()
   vm_create_disk()
   vm_extra_create_disk()

 - In tools/azure.conf, override:

   vm_extra_install_base()
   vm_extra_pre_umount()
   vm_extra_create_disk()

 - In tools/openstack.conf, override:

   vm_extra_install_base()
   vm_extra_pre_umount()

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