Fix undefined player variable error during server initialization

* Set ambient combat "center units" to an empty array instead of
referencing playable unit names. When some of those player slots weren't
filled, the units would be nil. Since the ambient combat script accepts
a "multiplayer" argument that automatically centers action on all live
playable units, the "center units" argument is not actually needed.
* When in debug mode, spawn ambient combat groups and paradrops every
ten seconds.