Fixed two bad xrefs in terminology/overcommit

Link to "Scheduler" needed explicit text
Link to community overcommit doc was poorly formed

Also deleted extraneous bullet from security-groups

terminology/identity-service -- fixed poorly formed link.

terminology/security-groups -- replaced link to gerrit with link
to sahara section in ops-guide

Change-Id: Id8bab877c38ced27dbb4f3dcf98fe41ad395504d