Update Ironic User Guide post landing for 41af7d6b

This patch is a post landing for 41af7d6b741a5a24e2cd212ab543b0b5be105551
Fixing:
Update logical architecture diagram
Brief description of Ironic service
Bullet points

Change-Id: I177041fa7db035d95a93e6345faa64177dbf888e
Merge "Update Ironic User Guide post landing for 41af7d6b"