FileStorage: abort creating over an existing file
FileStorage: abort creating over an existing file

In FileStorage there is a TODO:
 decide whether we should check for existing files and
 abort or not
After Ganeti ate my instance data I decided. Let's abort.
In general there is no reason we should overwrite existing files, and
doing it can be very harmful for preexisting files on the host.

Reviewed-by: iustinp


yiannist/ganeti,grnet/snf-ganeti,andir/ganeti,ganeti-github-testing/ganeti-test-1,ganeti-github-testing/ganeti-test-1,bitemyapp/ganeti,mbakke/ganeti,apyrgio/snf-ganeti,ganeti/ganeti,apyrgio/snf-ganeti,onponomarev/ganeti,mbakke/ganeti,yiannist/ganeti,yiannist/ganeti,grnet/snf-ganeti,dimara/ganeti,leshchevds/ganeti,apyrgio/ganeti,leshchevds/ganeti,andir/ganeti,onponomarev/ganeti,mbakke/ganeti,apyrgio/ganeti,leshchevds/ganeti,ganeti/ganeti,dimara/ganeti,bitemyapp/ganeti,ganeti/ganeti,andir/ganeti