Fix for "sh: 'rsync' no such file or directory" on RHEL-like systems

[legal90]

The ark resource requires rsync package to be installed.

It fixes the next error (reproduced on "centos6" and "stash" cookbook version 3.12.1):

...
==> centos6: Mixlib::ShellOut::ShellCommandFailed
==> centos6: ------------------------------------
==> centos6: execute[unpack /var/chef/cache/stash-backup-client-1.5.0.zip] (/tmp/vagrant-chef-3/chef-solo-1/cookbooks/ark/providers/default.rb line 55) had an error: Mixlib::ShellOut::ShellCommandFailed: Expected process to exit with [0], but received '127'
==> centos6:
==> centos6: ---- Begin output of unzip -q -u -o /var/chef/cache/stash-backup-client-1.5.0.zip -d /tmp/d20141111-3706-192msdu && rsync -a /tmp/d20141111-3706-192msdu/*/ /opt/atlassian/stash-backup-client-1.5.0 && rm -rf /tmp/d20141111-3706-192msdu ----
==> centos6:
==> centos6: STDOUT:
==> centos6:
==> centos6: STDERR: sh: rsync: command not found
==> centos6:
==> centos6: ---- End output of unzip -q -u -o /var/chef/cache/stash-backup-client-1.5.0.zip -d /tmp/d20141111-3706-192msdu && rsync -a /tmp/d20141111-3706-192msdu/*/ /opt/atlassian/stash-backup-client-1.5.0 && rm -rf /tmp/d20141111-3706-192msdu ----
==> centos6:
==> centos6: Ran unzip -q -u -o /var/chef/cache/stash-backup-client-1.5.0.zip -d /tmp/d20141111-3706-192msdu && rsync -a /tmp/d20141111-3706-192msdu/*/ /opt/atlassian/stash-backup-client-1.5.0 && rm -rf /tmp/d20141111-3706-192msdu returned 127...

[bflad]

LGTM since rsync is available as package on supported platforms and https://supermarket.chef.io/cookbooks/rsync seems a little overkill since it only installs the package as well. Maybe in the future this can be an attribute that people can override to not install either rsync or unzip via package if they don't want.