fix bug with SBD device on VirtualBox

The backslashes in the sed command were getting interpolated during
provisioning, not at runtime.
extract switch_to_qemu_if_required() function

This refactoring also fixes a bug.
extract batch_build_proposals()
heat demo: make VAGRANT_CONFIG_FILE externally tweakable
HA demo: make use of vagrant config file explicit
check VAGRANT_CONFIG_FILE is valid
make it easier to build HA demos with only 8GB of RAM