(maint) Use fully qualified paths for remote_ssh_cmd

Previously the remote_ssh_cmd utility shelled out to ssh when the fully
qualified path was available. This commit updates the implementation to
save the results of check tool and then use it. The tests are also
updated for this change and some stubs are made more specific.
(maint) Update all instances of rsync_to to Pkg::Util::Net.rsync_to
(maint) Update all instances of rsync_from to Pkg::Util::Net.rsync_from