Document Docker/dockerctl

Created LXC and Docker/dockerctl terminology articles
Created Docker section in ops guide. This includes some info
  about Fuel and system changes made to accomodate Docker;
  these probably belong in a general architectural description
  but since that doesn't exist now, I left them here.

Closes-Bug: #1314644

Change-Id: I0c885f43d2cbc88e5d9f47d35ec92acfa615c811