Rename OmnibusHelper function chef_status => service_up?

This better reflects the behavior which is to return true if the
specified service is running and false otherwise.
Check for postgres running as part of guards in postgresql recipe

Add PgHelper to organize guards used in the postgresql recipe. With
these changes we skip resources that create dbs and db users if the db
isn't running. If the db _is_ running, then we run checks to make the
resources idempotent.

This change allows the following command sequence to succeed:

    chef-server-ctl stop
    chef-server-ctl reconfigure
    chef-server-ctl start

Which is an important pattern for upgrades and for major config
changes, such as changing the port on which a service runs.