apiserver does not require a cloud provider or machine list

Currently the apiserver will not start unless a machine list or a
valid cloud provider is specified. This prevents a workflow that
manages machines solely through the minions API.

Fix the issue by changing the apiserver to only log a message that
the apiserver is being started with an empty machine list.

This patch results in a change in behavior. The apiserver will no
longer exit non-zero if a cloud provider or machine list is not
configured.
Merge pull request #1320 from kelseyhightower/run-without-machines

apiserver does not require a cloud provider or machine list