Add system_user and system_group parameters to params.pp.

[buzzdeee]

The last merge broke activemq for me on OpenBSD. There is no
user/group "activemq", on OpenBSD its "_activemq".
The change below is to fix the breakage for me ;)

Let me know if it needs enhancements/changes.

Set the parameter for Debian and the rest of the world to "activemq",
but for OpenBSD default to the package user/group "_activemq"

Allow the parameter to be overridden in the init.pp, and
push the values, and make use of them in config.pp.

[buzzdeee]

rebased to your current master

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