Add a note highlighting MongoDB disk requirements

Added a note to highlight the disk requirements of MongoDB
when database journaling is enabled (default). Provided
instructions for switching to smaller files and a link
to instructions for disabling journaling entirely.

Change-Id: Ie7410018ce664cddd66309df8d1b1bb17abdfcbb
Closes-Bug: #1253214
Add group information to nova config reference

Change-Id: I8f25f33e031fb28f26a773a79e99e719e671bc7e
Change "." to "/" in nova.flagmappings

This reflects a change in openstack-doc-tools to accomodate options
in at least Neutron that have "." in the option name.

Change-Id: Ic32308de4b156a8bb880a09dce72629fd058d8f6
Merge "Add a note highlighting MongoDB disk requirements"
Merge "Add group information to nova config reference"