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";

The `mysql_variable` module doesn't properly 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 it 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 the variable to the 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.

This is the patch originally referred to as 2/3 in #4581.