1.keep the column display order consist in extension list with
  and without "--long" option.
2.rework for network extentsion list, openstacksdk return object,
  so the logic should be same with other service.
3.add some unit test cases, like: extension list --network --long,
  extension list --network --compute, to cover regular use cases.
4.raise exact exception when network extension don't exist, avoid
  internal TypeError in "extension show" commands.

Change-Id: I2e23ced80d8da8aa1106b22472db850367b351ce
Closes-Bug: #1689233

Implement Neutron feature of Extension Show into
OpenStack Client.

Change-Id: Ifecb794838cb3bf8c2466d178345349db3cd4003
Implements: blueprint extension-show

this will better isolate the unit tests from the functional tests.
unfortunately, the "integration" tests had to be lumped into the
"unit" tests since we need the separation in testr.conf

Change-Id: Ifd12198c1f90e4e3c951c73bfa1884ab300d8ded