Move vpn unittest code to neutronclient.tests

Closes-Bug: #1217205

This commit fixes the following bugs which are detected by moving
VPN unittests.

* Raise CommandError instead of Value/KeyError in VPN parameter
  validation (Closes-Bug: #1219192)

* Delete dpd_timeout_less_than_interval test in IPsec site
  connection tests.  CLI does not validate this and it raises
  ConnectionError and results in the test failure.
  (Previously this test catches a general Exception,
   so the unit test does not fail unexpectedly.)
  (Closes-Bug: #1219389)

  There is a confusion on whether dpd_timeout == dpd_interval
  should be accepted in server side code, document and CLI.
  it is important to make VPN related tests enabled, so I decided
  to leave the fix of validation logic in the future work.
  (Filed as Related-bug: #1219440)

Change-Id: Ie4f4e8cdf533a983d0a6d4d564a7267a3323b832
Merge "Move vpn unittest code to neutronclient.tests"