Johnson Will Retire  As CACI's No. 2 Executive CACI International Inc.'s president of U.S. operations, L. Kenneth Johnson, will retire Nov. 1.  The Arlington government contractor said in a statement that the 58-year-old Johnson, its second in command, is leaving "to pursue personal interests."&lt;BR&gt;\&lt;FONT face="verdana,MS Sans Serif,arial,helvetica" size="-2"\ color="#666666"&gt;&lt;B&gt;-The Washington Post&lt;/B&gt;&lt;/FONT&gt;