[All] Use .swagger-codegen-ignore instead of writeOptional

I think .swagger-codegen-ignore is better since it can be configured by the end user whereas writeOptional files are hard-coded in the codegen. For instance, when generating the petstore samples, you want to overwrite the pom.xml but it doesn't occur unless you rm the file first. It would be better to pre-configure in the templates the .swagger-codegen-ignore to ignore pom.xml, build.gradle, etc... The only writeOptional file would then be .swagger-codegen-ignore