gpaddmirrors: Safely stop primaries

Shutting down the primaries using SIGKILL keeps the PID files around,
whereas SIGTERM more gracefully stops the primaries and removes the PID
files. When PID files hang around, it keeps future tests from being able
to create a cluster.

gpMgmt/test/behave/mgmt_utils/gpaddmirrors.feature

@@ -103,7 +103,7 @@ Feature: Tests for gpaddmirrors
         And gpaddmirrors adds mirrors with temporary data dir
         And an FTS probe is triggered
         And the segments are synchronized
-        When user kills all primary processes with SIGKILL
+        When user kills all primary processes with SIGTERM
         And user can start transactions
         Then verify that there is a "heap" table "public.heap_table" in "gptest" with "100" rows
         Then verify that there is a "ao" table "public.ao_table" in "gptest" with "100" rows