Added initial agentless check for MDADM RAID arrays.

This is an initial version, but it should be functional. It will work for one or multiple arrays. 
For now, it's using multiple "client.exec_command"s and grep/awk. It's probably much better to pull the contents of /proc/mdstat and do the comparisons locally, so I will update to that when I have time. 

The structure is based on the other check_X_by_ssh.py checks, for consistency. 
Currently, it returns "OK" if the raid is healthy, and "CRITICAL" if degraded. It also returns perf_data for recover(rebuilds) and check(scrubs). This perfdata is always at 100%, but will return 0-100% during a rebuild or scrub. I think the perfdata is useful, so that you can know how long a rebuild or scrub takes. I'm not sure if this is the best way to report it, so any advice is welcome.