Set firewall_rules only after appending all rules

The firewall_rules attribute was being re-set per each firewall rule.
Instead if should only be set once when all firewall rules are appended
to _firewall_rules.

A minor PEP E128 (continuation line under-lined) is also being fixed
here.

Change-Id: I9db6d3dc0ad11a3194513d80f15151c29aa019cd
Closes-Bug: 1328625
Ensure .status_code is defined for all NeutronClientExceptions

nova contains plenty of code like:
  except neutron_client_exc.NeutronClientException as e:
      if e.status_code == 409:
          ...

This change declares a fallback of 0 for exceptions without a more
explicit status code.  This was the behaviour prior to Change-Id
I99b9560b3afaf5884fd00353323267da450338fa

Change-Id: Ib736205066fd5cd9738fd3fb3b64f6590803cf39
Improve help strings

Several changes to make help strings consistent:
* End options with "."
* Add missing space between lines
* Capitalize first word of help strings
* Improve wording of text
* Make strings consistent.
* Use "VPN service"

Closes-Bug: #1321871

Change-Id: I8fdc356574d680c222b8a26dcc5ad8e3a2b4e4f1
Merge "Ensure .status_code is defined for all NeutronClientExceptions"
Merge "Set firewall_rules only after appending all rules"
Merge "Improve help strings"