vm-img: wait longer that the VM boots

Currently, NEO-STRESS often fails because it is assigned
on the same test node as other test suites that:
- overload the test node;
- are often triggered at the same time.
1 job for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-Master

22:19:25