Wait until Nailgun's API is ready after VM reset

After resetting Fuel master node we wait until 8000/tcp
port (nginx) is accepting connections. We also should wait
up to 2 minutes until Nailgun's API is ready.

Change-Id: I2360e0dab68ae0a20618f1da64ee5d6b4499e126
Closes-bug: #1368537
Merge "Wait until Nailgun's API is ready after VM reset"