Improve docs about horizon settings

added ALLOWED_HOSTS
added SESSION_TIMEOUT

Since settings become longer and longer, I re-arranged the options
in alphabetical order.

Change-Id: I4424cc178ec0cd7892b0f07cb0192b5af5b2ee0d
Partial-bug: #1221115
Remove "Delete" buttons when VPN objects are in use

Deleting attached services and policies doesn't
cause NotImplementedError anymore. Moreover,
options "Delete VPN Service", "Delete IKE Policy" and
"Delete IPSec Policy" don't appear in corresponding row
if there is a connection created using these objects.
Some tests are added to detect such errors in future.

Fixes bug 1252330

Change-Id: I9ed1b4ecdad54b89bef45b2f5e422c30447cceb5
Fixes some typos spotted in a few Horizon files.

It fixes a few typos spotted in comments and debug/exception messages of these Horizon files:
 horizon/tables/base.py
 horizon/tables/formset.py
 horizon/tables/views.py
 horizon/utils/functions.py
 horizon/workflows/base.py

Change-Id: I7f3069df3f60a6a788f71870f9b49ffe23fdb5a6
Closes-Bug: #1268712
Merge "Improve docs about horizon settings"
Merge "Fixes some typos spotted in a few Horizon files."
Merge "Remove "Delete" buttons when VPN objects are in use"