[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>`

Author: tzolov <christian.tzolov@gmail.com>

Closes #1531 from tzolov/SPARK-2619 and squashes the following commits:

6d95343 [tzolov] [Build] SPARK-2619: Configurable filemode for the spark/bin folder in the .deb package