Update developer quickstart doc about required OS version
Update developer quickstart doc about required OS version

Minimum required distro support has changed for DevStack:

Ubuntu - 16.04 or later
Fedora - 24 or later

Updates the Ironic developer documentation to reflect that.

Change-Id: I983d9a7e586aeee03122c53fe42ea83b3e3028bd
Closes-Bug: #1681524

SauloAislan/ironic,pshchelo/ironic,openstack/ironic,SauloAislan/ironic,pshchelo/ironic,openstack/ironic