Improve handling of ssh key path in instance_ssh()

[rawiriblundell]

As per #324 , this PR proposes a tiny change to the handling of the ssh key path within the instance_ssh() function.

The change is literally from this:

-i "${BMA_SSH_DIR:-~/.ssh/}$keyname"

to this

-i "${BMA_SSH_DIR:-~/.ssh}/$keyname"

The rationale is that if BMA_SSH_DIR is missing a trailing slash, this will ensure that one is present. If BMA_SSH_DIR does have a trailing slash, then a non-fatal double-slash will be present (i.e. /path/to/BMA_SSH_DIR//keyname) and the connection will still work. Either way, the default behaviour of ~/.ssh/$keyname remains intact.

With the current code, a BMA_SSH_DIR without a trailing slash results in e.g. /path/to/BMA_SSH_DIRkeyname which will obviously fail.

[mbailey]

Thanks Rawiri!