[Build] SPARK-2619: Configurable filemode for the spark/bin folder in debian package

Add  a `<deb.bin.filemode>744</deb.bin.filemode>` property to the `assembly/pom.xml` that defaults to `744`.   
Use this property for ../bin folder <filemode>. 

This patch doesn't change the current default modes but allows one override the modes at build time: 
`-Ddeb.bin.filemode=<new mode>`