Document Amazon Block Device Mapping fields

builder/docker: support custom communicators

core: add build_name and build_type functions

builder/qemu: use proper ssh port [GH-2074]

builder/virtualbox,vmware: validate output dir in step

fix: virtualbox rename fixes overrides [GH-1828]

template: allow _ prefix to root level keys for comments [GH-2066]

provisioner/shell: escape single quotes [GH-2067]

command/fix: validate resulting template [GH-2075]

config file doesn't need to exist if set [GH-2225]

builder/openstack: support user data [GH-1867]

builder/openstack: load extensions, stop server if supported

Add EBS snapshot tags #846 #1219

update GH issue for the `force_deregister` option

builder/amazon: Add force_deregister option

Windows AWS instances