disk/api.py: refactors extends and adds missing tests

The method extend does not log any information if there is a
problem during the resize with "resize2fs".

This patch refactor extend and adds missing tests.

Change-Id: I81079a79e0449381440f202f6143b48f96453a11
Related-Bug: #1270086
Merge "disk/api.py: refactors extends and adds missing tests"