Mocks utils.execute_with_timeout for mongodb tests

The mongodb test_prepare_from_backup unit test doesn't
mock utils.execute_with_timeout causing it to prompt
for a password and/or hang.

Change-Id: I22d839f9bbd6faf97bba16616e7f2cbcf725ab1e
Closes-Bug: 1353570
Merge "Mocks utils.execute_with_timeout for mongodb tests"