Allow ERB templates to be tolerant of a leading BOM

See additional details in MODULES-1996 / PUP-6447.

The basic gist is that internal calls to read, like the one made by template loading with Puppet::FileSystem.read_preserve_line_endings, do not specify the "allow bom" flag when reading UTF-8 files.

This should be a straightforward addition that allows templates (for instance PowerShell templates that were once PS1 files created by PowerShell ISE that have a leading BOM) to properly remove the leading BOM.