Prevent the instance to hang at boot if the FS was corrupted

e.g. :

/dev/vda1: recovering journal
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
Free blocks count wrong (1013426, counted=679344).
Fix<y>?
Merge pull request #1 from pgrandin/patch-1

Prevent the instance to hang at boot if the FS was corrupted