Changed esx_mem ssh connection to no longer store known_host data

Due to Net::SSH being used to send a request to restart hostd service for the esx mem install, known_host data was being kept on the appliance.  This causes issues when the same hypervisor IP is used later with a different server, since the SSL fingerprint won't match up to what is in the appliance's known_hosts file.

This fix sets StrictHostChecking to off, and also sets it so that known_host data is not being written anywhere (/dev/null).

Here's some reference links:
http://net-ssh.github.io/ssh/v2/api/classes/Net/SSH.html
http://stackoverflow.com/questions/3579508/need-to-ignore-hostkeymismatch-when-logging-in-using-netssh-start