Update heat-agents setup files

Update heat-agents setup files

This patch moves the image pull step out of the service heat-agent
service script to ease the service init process and to make it more
reliable. By doing this outside of the service script, it's possible to
know when the `firstboot` script failed and report back.

It also updates the firstboot yaml file to point to the
`tripleoupstream` org.

Co-Authored-By: Flavio Percoco <flavio@redhat.com>
Co-Authored-By: Martin André <m.andre@redhat.com>
Change-Id: I2f0b8092ec69320ee370e1d7d20b8c15c95a1d0d