add CLEAR cqlsh command

patch by Paul O'Fallon; reviewed by Stefania for CASSANDRA-10086

CHANGES.txt

@@ -1,4 +1,5 @@
 2.2.1
+ * add CLEAR cqlsh command (CASSANDRA-10086)
  * Support string literals as Role names for compatibility (CASSANDRA-10135)
  * Allow count(*) and count(1) to be use as normal aggregation (CASSANDRA-10114)
  * An NPE is thrown if the column name is unknown for an IN relation (CASSANDRA-10043)

bin/cqlsh.py

@@ -242,7 +242,9 @@ def find_zip(libprefix):
     'expand',
     'paging',
     'exit',
-    'quit'
+    'quit',
+    'clear',
+    'cls'
 )
 
 
@@ -275,6 +277,7 @@ def registrator(f):
                    | <expandCommand>
                    | <exitCommand>
                    | <pagingCommand>
+                   | <clearCommand>
                    ;
 
 <describeCommand> ::= ( "DESCRIBE" | "DESC" )
@@ -362,6 +365,9 @@ def registrator(f):
 <exitCommand> ::= "exit" | "quit"
                 ;
 
+<clearCommand> ::= "CLEAR" | "CLS"
+                 ;
+
 <qmark> ::= "?" ;
 '''
 
@@ -2170,6 +2176,16 @@ def do_exit(self, parsed=None):
             self.conn.shutdown()
     do_quit = do_exit
 
+    def do_clear(self, parsed):
+        """
+        CLEAR/CLS [cqlsh only]
+
+        Clears the console.
+        """
+        import subprocess
+        subprocess.call(['clear','cls'][myplatform == 'Windows'], shell=True)
+    do_cls = do_clear
+
     def do_debug(self, parsed):
         import pdb
         pdb.set_trace()