Make MySQL query logging optional

* lib/databases/mysql: Wrap query log configuration in a check for a
ENABLE_QUERY_LOGGING variable.

* stackrc: Add the DATABASE_QUERY_LOGGING variable defaulted to True.

Change-Id: Iddf8538ad0a1e36e2c6944dc70315984026c8245
diff --git a/stackrc b/stackrc
index 49fb26b..276ce33 100644
--- a/stackrc
+++ b/stackrc
@@ -59,6 +59,9 @@
     source $RC_DIR/.localrc.auto
 fi
 
+# This can be used to turn database query logging on and off
+# (currently only implemented for MySQL backend)
+DATABASE_QUERY_LOGGING=$(trueorfalse True $DATABASE_QUERY_LOGGING)
 
 # Repositories
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