Use SecureString where there is data to be cleansed

SecureString has quite a few places it should be used in the user
management code.

Tested: ran set password, test password, and other commands

Change-Id: Ia53bc914d25f7965c3e72d5cf18346e0fa9339b9
Signed-off-by: Vernon Mauery <vernon.mauery@linux.intel.com>
diff --git a/user_channel/user_mgmt.hpp b/user_channel/user_mgmt.hpp
index 6dbda25..833f6a1 100644
--- a/user_channel/user_mgmt.hpp
+++ b/user_channel/user_mgmt.hpp
@@ -241,7 +241,7 @@
      *  @return ccSuccess for success, others for failure.
      */
     Cc setSpecialUserPassword(const std::string& userName,
-                              const std::string& userPassword);
+                              const SecureString& userPassword);
 
     /** @brief to set user privilege and access details
      *