Prior to this commit, if you specified a `version` for the salt provider
but no `install_type` Vagrant would fail to pass the proper parameters
to the bootstrap install script. This commit fixes that by adding some
validation to the salt provider if `version` is specified but not
`install_type`. It also adds some extra context for certain config
validation error messages so that the user knows what option was
incorrect, rather than the message just referring to the option as
*this*.
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