add ppa config option

[kwmonroe]

Two use cases:

1 - restricted networks
Let users set this to a private ppa in case they can't get to openjdk from the archives.

2 - openjdk-8 on trusty
Trusty only includes openjdk-6 and 7 in the archives. If people want to use this charm to install openjdk-8 on trusty, they would set this option to ppa:openjdk-r/ppa.

[kwmonroe]

point 2 is a bit m00t now that cory adds the openjdk-r ppa on trusty.. but still, this would be nice.