Spelling updates to openstack-security-guide.

Fixes a number of minor spelling issues picked up by running the aspell
en-US dictionary against the guide. Note that although the en-US
dictionary was used I have shied away from fixing issues where the only
cause of issue is en-GB V en-US but rather stuck to what I consider
appear to be clear mistakes.

Change-Id: I0bf73d31edbd0e8c42387716b4358934c78f7306