Rename NativeEosService to DataCenterControl

This is a wee bit more descriptive name. While we are here, also ditch
java.lang.Void, as it promotes nulls. Use yamg.common.Empty, which has a
singleton instance.

Fold common implementation code into a static method, which is making
things very obvious.

Change-Id: I28230523034224e978244d1edce997376812bf91
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>