Fix --dump_python option in cmsDriver

[mark-grimes]

Currently, if you specify the --dump_python option to cmsDriver the resulting file will have none of the era changes applied. When execfile is used on the confg file during the if options.dump_python: branch, it picks up references to the modules already loaded (which don't have eras applied) rather than reloading any modules.
This fixes that by switching on any eras before the first reading of the modules.

@lveldere, @Dr15Jones

[cmsbuild]

A new Pull Request was created by @mark-grimes (Mark Grimes) for CMSSW_7_6_X.

Fix --dump_python option in cmsDriver

It involves the following packages:

Configuration/Applications

@cmsbuild, @franzoni, @davidlange6 can you please review it and eventually sign? Thanks.
@ghellwig, @Martin-Grunewald this is something you requested to watch as well.
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@Degano you are the release manager for this.
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