Define root_helper variable under the [AGENT] section

Fixes bug 1105523

The patch set adds 2 new functions:
1. register_root_helper - this enables all wanting to use
the root_helper variable. This is under the section AGENT.
2. get_root_helper - this is a helper function that returns the
root_helper. This should be used when the application used to
have the root_helper defined under the section DEFAULT. This
ensures backward compatability.

Change-Id: Iba72c4fc89ba5329ea045483287012f82b306250