remove default value for keyfile: ${user} is the remote user, not local

[koollman]

/home/${user}/.ssh/id_rsa may not exist if $user does not exist locally.
It could also not be the right choice, depending on ssh configuration and $execuser.
If no key file is specified, let ssh find the right one.

[eputnam]

@koollman thanks for the PR! Could you please have a look at the puppet-lint warnings in travis? https://travis-ci.org/puppetlabs/puppetlabs-rsync/jobs/278679330

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