fix(deisctl): properly resolve ~ in path

While `$HOME` is resolved properly in paths, `~` is not
and requires special handling.

I tested this with the following, on both bash and zsh:
```console
$ deisctl config platform set sshPrivateKey=$HOME/.ssh/deis
$ deisctl config platform set sshPrivateKey=~/.ssh/deis
$ deisctl config platform set sshPrivateKey=/Users/carmstrong/.ssh/deis
$ DEISCTL_UNITS=/Users/carmstrong/gocode/src/github.com/deis/deis/deisctl/units deisctl refresh-units
$ DEISCTL_UNITS=$HOME/gocode/src/github.com/deis/deis/deisctl/units deisctl refresh-units
$ DEISCTL_UNITS=~/gocode/src/github.com/deis/deis/deisctl/units deisctl refresh-units
```

fixes #2677
It is necessary because the default value for `refresh-units --path` is literally `$HOME/.deis/units`; see https://github.com/deis/deis/blob/master/deisctl/cmd/cmd.go#L626. That default string won't be expanded by the shell if the user doesn't supply a `--path` value.