Add force option to nuxeoctl start

The nuxeoctl start return no error code when an instance is already running, this is compliant with LSB.

Adding a -f or --force option will enable to get an error in this case because a new instance can not be started. The nuxeoctl start should return an error code 6 (program is not configured) because of http port binding conflict.

This option will be useful for CI, the job will stop immediately if there is an already running server on the slave and will not give a wrong feedback.