Update fabfile.py
Update fabfile.py

Adds support for a persistent user volume.
General code-cleanup.
Support for listing ubuntu AMIs.

bearecinos/oggm,OGGM/oggm,TimoRoth/oggm,OGGM/oggm,juliaeis/oggm,juliaeis/oggm,TimoRoth/oggm,anoukvlug/oggm,anoukvlug/oggm,bearecinos/oggm