Munki staging processor

[choules]

This pull request introduces a new shared processor which may be used to implement an automatic staging of items in a Munki repo.

To use this processor, the wycomco-recipes repo should be in AutoPKG's search path and then the processor may be referenced like this:

<dict>
    <key>Processor</key>
    <string>com.github.wycomco.SharedProcessors/MunkiAutoStaging</string>
</dict>

To enable this reference method, I have created a stub recipe as describe in the AutoPKG wiki.

The processor uses common input vars from a Munki recipe, and the following new ones

• MUNKI_STAGING_CATALOG: The name of the staging catalog, defaults to "testing"
• MUNKI_PRODUCTION_CATALOG: The name of the production catalogs, defaults to "production"
• MUNKI_STAGING_DAYS: The amount of days to keep a new item in staging catalog

Currently, there is no way to force a makecatalog using the MakeCatalog.munki recipe, see this issue for details. So it may be the case, that even though the catalog was set correctly inside the pkginfo file, the catalogs do not get updated correctly.

[choules]

Updated to reflect the comments above.

[choules]

Tested locally, everything works as expected.