packer: Template understands variables, puts it into a Build
packer, packer/rpc: Update Build interface to allow variable overrides
packer: properly override/send user variables to prepare
common: PackerConfig understands user vars
common: UserData => UserVars
builder/*, provisioner/*: set user vars
provisioner/salt-masterless: convert to new template stuff + user vars
command/build,validate: compiles again with latest user var changes
packer: add ParseTemplateFile since that is useful
common/command: introduce this package for common cmmand stuff
command/validate: accept -only/-except and use new common stuff
command/build,validate: use new ParseTemplateFile methods
common/command: rename BuildFilters to BuildOptions
common/command: parse the "-var" flag
command/build,validate: pass user vars to Prepare
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