common: downloads can be HTTPS [GH-587]
packer: builder prepare can return warnings
packer/rpc: implement new warnings interfaces
packer/rpc: use packer.MockBuilder for tests
packer/rpc: test warnings with builders
packer/plugin: new Builder interface
builder/amazon/*: warnings
builder/all: update to warnings
command/validate: output warnings
command/build: get command passing
builder/virtualbox,vmware: warning if shutdown_command is not specified
command/build: output warnings
Merge branch 'f-builder-warnings'
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