Fix some typos in architecture doc

As a new person on this project, I found this document to be excellent.
Fixed a few typos found while reading:
Change where we refer to Havana in the future tense.
Capitalize Ceilometer when referring to the project.
Other small typos

Change-Id: I2fc422950538135fbcf38c468401b4a5cf8640f2
Merge "Fix some typos in architecture doc"