Allow to define an alternative pomFile when sign (gpg:sign) the project artifact

This small modification allow to specify an alternative pomFile to sign instead of the default project file. It's very useful when a custom pom file has been generated during the build process. I'm using a complex build configuration for a framework where I need to create custom pom files. I'm able create a release from my CI and publish it to the central repository the gpg:sign signs the wrong pom file.