OpTestSystem: Improve petitboot_exit_to_shell
OpTestSystem: Improve petitboot_exit_to_shell

During BootTorture test execution, we failed to exit to the shell.
So this patch improves it by re-trying for three times when timeout
happens

Signed-off-by: Pridhiviraj Paidipeddi <fd8ea14509c256cbd04eaa4ee745a213945e70a1@linux.vnet.ibm.com>

open-power/op-test-framework,open-power/op-test-framework,open-power/op-test-framework