[docs] Explain minimum disk requirements for MongoDB

[Fabrizio Soppelsa]

Fuel complains if cannot allocate enough space for the MongoDB partition, I think this should be documented somewhere in the Planning Guide (maybe), as a requirement.

Error in a HA environment with controllers+mongo:

Node 'Untitled (0c:fa)' has insufficient disk space
Volume group 'MongoDB' requires a minimum of 10240MB

[Irina Povolotskaya (ipovolotskaya)]

I've found some information on this issue here http://docs.mirantis.com/openstack/fuel/fuel-5.1/planning-guide.html#ceilometer-and-mongodb
Should some information on MongoDB be added here?

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/222392

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/222392
Committed: https://git.openstack.org/cgit/stackforge/fuel-docs/commit/?id=c7076d59df771953b8201042cfbcf5ed44d3131a
Submitter: Jenkins
Branch: master

commit c7076d59df771953b8201042cfbcf5ed44d3131a
Author: Svetlana Karslioglu <email address hidden>
Date: Wed Sep 9 22:30:41 2015 -0700

    Added minimum disk requirements for MongoDB

    Added a minimum of 10240 MB disk space as a requirement for a
    MongoDB partitition.

    Broke the section on more modular chunks. Rewrote the text according
    to the OpenStack Style standards.

    Closes-bug: #1380565
    Change-Id: I8bc051b8578c81772eeae91d0c47905eb6b9bb08