feature: allow downloading of package by remote host

[travisdowns]

Currently node_exporter install always downloads the package on the localhost and unpacks it there then uploads it to the remote host.

In the case the ansible controller is at the other end of a thin network pipe, or if it may not even have access to the package URL, it may be desirable to download and unpack it on the remote host instead.

This change adds that ability as an optional behavior to _common/install and wires it up for use by node_exporter, using the node_exporter_localhost_download variable. The default for node exporter and any other binaries remains the same as before: delegate to localhost.

Fixes #389.