Allow to change executable (shell/interpreter) when using raw

This patch adds an optional 'executable=' option to the raw command line to override the default shell (/bin/sh), much like the shell module does.

library/raw

@@ -4,7 +4,16 @@ DOCUMENTATION = '''
 ---
 module: raw
 short_description: Executes a low-down and dirty SSH command
-options: {}
+options:
+  free_form:
+    description:
+      - the raw module takes a free form command to run
+    required: true
+  executable:
+    description:
+      - change the shell used to execute the command. Should be an absolute path to the executable.
+    required: false
+    version_added: "1.0"
 description: 
      - Executes a low-down and dirty SSH command, not going through the module
        subsystem. This is useful and should only be done in two cases. The