more thoroughly skip host key checks

[justone]

Without this change, when the host key changes, you still get a big scary warning, even though the ssh connection succeeds. This just /dev/nulls the known hosts file so that nothing is shown.

Also, it adds a short option (-d) for skipping the host key check.

[simcap]

Two quick comments:

• -d flag we can do until we need a global -d flag, so no problem there
• instead of /dev/null we should use the Golang provided os.DevNull so that we stay compatible across platforms.

More generally, I am wondering if doing that we could hide and change the behavior of the SSH doing the connection. Because with the flag -d we just say that we disable the strict host key check (-o StrictHostKeyChecking) as per SSH spec, and nothing would indicate here that we redirect the warning to '/dev/null', which is done with -o GlobalKnownHostsFile and -o UserKnownHostsFile.

Therefore shouldn't we add another new flag (with a short version) like --ignore-warning or --ignore-khosts so that we specify the full behaviour wanted as for instance:

awless ssh myinstance ---ignore-khosts -d

[justone]

Very good point. I like the idea of separating the flag out. I'll update the PR.