More updates for Juno

I updated the installation guide for Juno as follows:

1) Unifed RHEL/CentOS/Fedora steps to use systemd through
   networking chapter. Will work on other chapters in a
   future patch.
2) Split openSUSE steps to use systemd.
3) Removed database configuration from nova on compute
   nodes (yay!).
4) Added workaround for RDO changing the neutron default
   "auth_strategy=keystone" back to "auth_strategy=noauth".
5) Re-add workaround for broken RDO neutron init scripts. See
   bug #1375746 for more information.
6) Applied other changes for RHEL/CentOS 7.
7) Improved nova-network content.
8) Other minor fixes.

Change-Id: I0a130580764f61544f5abebdde21b677a4d0ece9
Implements: blueprint installation-guide-improvements
Closes-Bug: #1287874
Closes-Bug: #1373367
Merge "More updates for Juno"