Added note to os_command_line

Added note that the os_command_line key is only valid
for AWS-type images.

Partial-Bug: #1307272

Change-Id: Ide43f1be5d09cdf11e1ae842cf2ac3e6d30b7ad3
Install Guide: Trove edits

Follow our conventions:
* Wrap long lines in screens.
* Use all-caps for variables.
* Use # su instead of $ sudo su like in other chapters

Change-Id: Iab395ada34583d24bc68db9c297a36b4432fbf20
Merge "Added note to os_command_line"
Merge "Install Guide: Trove edits"