HPCC-11902 Allow targets shown in WsEcl to be restricted through config

This will allow individual WsEcl instances to be set up for individual,
or small sets of target clusters in environments with many targets
available.

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Is it clear that having a blank target list will allow all targets?  Is the a tooltip or comment we can update to make it clear.  Should the title be "Target Restiction" or something similar.