slapos softwaretype: fix parse error on '+ =' when using buildout 2

Add a class SlapConfigParser which overrite ConfigParser.write method to fix parse problem when configuration like: [section] foo += bar

is included in buildout file. softwaretype recipe will generate buildout file with foo + = bar because ConfigParser doesn't reconize += delimiter and read key as "foo +", value as "bar". Then ConfigParser.write method generate

[section] foo + = bar ...

This is invalid with buildout version 2