Allow panel plugins to use ADD_EXCEPTIONS and UPDATE_HORIZON_CONFIG

This change allows panel plugins tell horizon which exceptions to treat as recoverable.
Previously only dashboard plugins were allowed to register exceptions.
Since there are now more settings that are common than specific to pluggable use case,
the doc was restructured to describe in general pluggable settings and then describe
the specific use cases.

Change-Id: I013ece33ae2b979e92be9c7ba50182025a1ecb91
Closes-bug: #1332688
Merge "Allow panel plugins to use ADD_EXCEPTIONS and UPDATE_HORIZON_CONFIG"