Check if OS image exists on the node before dumping
Check if OS image exists on the node before dumping

Signed-off-by: Jose A. Lopes <0fb9c4f8511df4ec2ae28af879e9d462f6218a2c@google.com>
Reviewed-by: Hrvoje Ribicic <be5828fca31f89218048bec4189e30a581aa65da@google.com>

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