update minAllowedPort of gpaddmirrors

Because the default offset is 1000, the default min port is 5432
（primary port + offset = mirror database port）
so， the default minAllowedPort should be 6432

minPort = min([seg.getSegmentPort() for seg in gpArray.getDbList()])

gpMgmt/bin/gppylib/programs/clsAddMirrors.py

@@ -455,7 +455,7 @@ class GpAddMirrorsProgram:
         """
 
         maxAllowedPort = 61000
-        minAllowedPort = 7000
+        minAllowedPort = 6432
 
         minPort = min([seg.getSegmentPort() for seg in gpArray.getDbList()])
         maxPort = max([seg.getSegmentPort() for seg in gpArray.getDbList()])