Improve contributor documentation

Fix various errors in the contributor documentation:

- Factual mistakes (wrong file locations, etc)
- Formatting errors
- Typos

Change-Id: I4863ca10a532ac74491cfb19f8382e3d5287d2f3
Hide key field in edit Flavor Extra Specs dialog

Nova allow user to add key/value to flavor, but its api is only
able to update value according to key.

Hide the key field in edit dialog, in order to avoid confusion.
User can delete and create new one when need to update "key".

Change-Id: Ibd13bfe8a3832d019d16a39b40f643b860e3c612
Closes-Bug: #1257882
Specify tenant_id when retrieving LBaaS/VPNaaS resource

In Neutron API, resources from all tenants are listed when
retrieving a list of resources with admin role. Horizon
project dashboard is for project-specific operations,
so we should retrieve only resources of the given project.
LBaaS/VPNaaS panels have this problem, but FWaaS does not.
router_list should be called with tenant_id filter too.

This commit also fixes VPN/FWaaS tests so that it extract tenant_id
from request attribute rather than use tenant_id of test_data.

Change-Id: I080e6c6d1e426a878c1d088763bbc7e5add15820
Closes-Bug: #1244126
Merge "Specify tenant_id when retrieving LBaaS/VPNaaS resource"
Merge "Hide key field in edit Flavor Extra Specs dialog"
Merge "Improve contributor documentation"