Make --storage option required for scheduler cli

[olegkovalenko]

MOTIVATION: Currently, if the --storage flag is missing it will default to file, which shouldn't be ever used in production environments. To avoid situations when due to default value scheduler writes its state to file and after failover node list are empty, it would be great to make the storage flag required, so the case where you forget so set a highly available state storage is impossible and setting file storage for development purposes would be explicit.

PROPOSED CHANGES: scheduler CLI make option --storage required, when it is not set user will see:

Option (* = required)  Description                           
---------------------  -----------                           
...

* --storage            Storage for nodes state. Examples:    
                         file:dse-mesos.json; zk:master:     
                         2181/dse-mesos; cassandra:9042:     
                         master.                             
...

Error: Missing required option(s) [storage]

RESULT: better user experience

[dmitrypekar]

Merged. Thank you.

[olegkovalenko]

Thanks!