Binary install mode for node_exporter and pg_exporter

[Vonng]

Scenario

Sometimes you may want ADD pigsty monitoring system to existing database cluster instead of provisioning a new one.
In that case. It is much convenient to install node_exporter & pg_exporter via copy binaries. (Directly from meta node instead of setup infra, download and install rpm, blahblah)

Proposal

Add a exporter_binary_install option. If set, node_exporter and pg_exporter are copied to /usr/bin/ directly.
But you'll have to put those two binary files in files directory.

[Vonng]

Fixed by 1d830af