OpTestOpenBMC.py: increase wait_bmc() timeout
OpTestOpenBMC.py: increase wait_bmc() timeout

Swift systems are taking more than 10 minutes between poweroff/poweron.
Increase the timeout to 15 minutes.

Signed-off-by: Klaus Heinrich Kiwi <d568b8cd7ef64877891488bb3328757d2d21c847@linux.vnet.ibm.com>

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