Add support for string values

The SET GLOBAL statement requires properly quoting of values. For example, the
following correct queries will fail if quotes are toggled:

mysql> SET GLOBAL innodb_lru_scan_depth = 2000;
mysql> SET GLOBAL master_info_repository = "TABLE";

`mysql_variable` module doesn't quote the value argument, therefore
string values will fail.

  # this task will pass, 2000 is passed without quotes
  - name: set a numeric value
    mysql_variable: variable=innodb_lru_scan_depth value=2000

  # this task will fail, TABLE is passed without quotes
  - name: set a string value
    mysql_variable: variable=master_info_repository value=TABLE

With this patch prepared statements are used. Proper quoting will be
done automatically based on the type of the variables thus an attempt
to convert to int, then to float is done in first place.

Booleans values, ie: ON, OFF, are not specially handled because they
can be quoted. For example, the following queries are correct and
equivalent, they all set _innodb_file_per_table_ to logical _True_:

mysql> SET GLOBAL innodb_file_per_table = "ON";
mysql> SET GLOBAL innodb_file_per_table = ON;
mysql> SET GLOBAL innodb_file_per_table = 1;

Tested in mysql 5.5 and 5.6.

library/database/mysql_variables

@@ -76,14 +76,48 @@ else:
     mysqldb_found = True
 
 
+def typedvalue(value):
+    """
+    Convert value to number whenever possible, return same value
+    otherwise.
+
+    >>> typedvalue('3')
+    3
+    >>> typedvalue('3.0')
+    3.0
+    >>> typedvalue('foobar')
+    'foobar'
+
+    """
+    try:
+        return int(value)
+    except ValueError:
+        pass
+
+    try:
+        return float(value)
+    except ValueError:
+        pass
+
+    return value
+
+
 def getvariable(cursor, mysqlvar):
     cursor.execute("SHOW VARIABLES LIKE '" + mysqlvar + "'")
     mysqlvar_val = cursor.fetchall()
     return mysqlvar_val
 
+
 def setvariable(cursor, mysqlvar, value):
+    """ Set a global mysql variable to a given value
+
+    The DB driver will handle quoting of the given value based on its
+    type, thus numeric strings like '3.0' or '8' are illegal, they
+    should be passed as numeric literals.
+
+    """
     try:
-        cursor.execute("SET GLOBAL " + mysqlvar + "=" + value)
+        cursor.execute("SET GLOBAL " + mysqlvar + " = %s", (value,))
         cursor.fetchall()
         result = True
     except Exception, e:
@@ -203,11 +237,14 @@ def main():
     else:
         if len(mysqlvar_val) < 1:
             module.fail_json(msg="Variable not available", changed=False)
-        if value == mysqlvar_val[0][1]:
+        # Type values before using them
+        value_wanted = typedvalue(value)
+        value_actual = typedvalue(mysqlvar_val[0][1])
+        if value_wanted == value_actual:
             module.exit_json(msg="Variable already set to requested value", changed=False)
-        result = setvariable(cursor, mysqlvar, value)
+        result = setvariable(cursor, mysqlvar, value_wanted)
         if result is True:
-            module.exit_json(msg="Variable change succeeded", changed=True)
+            module.exit_json(msg="Variable change succeeded prev_value=%s" % value_actual, changed=True)
         else:
             module.fail_json(msg=result, changed=False)