Expand environment variables when resolving config values in PackageTool

[HowardWolosky]

Current guidance given to users is that if any of the config values needs to reference environment variables, that they should not be added to the config, but should rather be specified at the command-line. There isn't any reason for this though. Resolve-PackageParameters can be updated to call [Environment]::ExpandEnvironmentVariables() to resolve any config value that has environment variables specified.