Do not get parser directly from ShowOne in fuel2 client

[Sebastian Kalinowski]

BaseShowCommand in fuel2 commands has a default parser that seems not to be useful. In few commands that inherits from it, the parser gets overwritten and the params added in BaseShowCommand are removed in favor to the one from original cliff ShowOne command.

Let's add a method called like "get_base_parser" that will be a helper to get the "raw" parser from ShowOne class instead of doing it by hand in those places:

https://github.com/stackforge/python-fuelclient/blob/55bd33a15875bfbe0790f9d48c8f7697312b822a/fuelclient/commands/environment.py#L57-L58
https://github.com/stackforge/python-fuelclient/blob/55bd33a15875bfbe0790f9d48c8f7697312b822a/fuelclient/commands/environment.py#L140-L141

There are also incoming reviews where the same thing is happening, like this one:
https://review.openstack.org/#/c/207398/14/fuelclient/commands/network_group.py (line 93, line 130)

[Dmitry Pyzhov (dpyzhov)]

We passed SCF in 8.0. Moving the bug to 9.0.