Fix install of `newrelic_plugin_agent[pgbouncer]` python package

On Ubuntu 12.04 (and potentially others), trying to install
`newrelic_plugin_agent[pgbouncer]` breaks b/c it cannot find
`pg_config`.

Turns out either `libpq-dev` (for Debian/Ubuntu systems) or
`postgresql-devel` (others) is needed for this to succeed. See
http://stackoverflow.com/a/12037133/133479 for details.
install dependencies only when required

this moves installation the following dependencies to the newrelic_ng_plugin_agent lwrp
- newrelic_plugin_agent[mongodb]
- newrelic_plugin_agent[pgbouncer]
- newrelic_plugin_agent[postgresql]
- libpq-dev resp. postgresql-devel
fix hardcoded pidfile path in debian init script