Do not download RNG Schema, instead use local copies

Using local copies instead of remote ones speeds up the process
significantly since we do not need to download everytime more than
600 kb of data. The additional benefit is that testing now works
also while you're offline, no need to do any internet connection
for it.

The files change seldom, so it is fine to add them.

Tested with:
 time openstack-doc-test --api-site --check-syntax

Before this change:
real	0m4.020s
user	0m0.212s
sys	0m0.047s

After this change:
real	0m0.164s
user	0m0.142s
sys	0m0.022s

Change-Id: I541e75580ae734ccd5d48d699eb48225203646e8
Add parameter --ignore-dir

This allows to ignore directories, e.g.
ignore in compute api the incubation and v1 dirs with:

openstack-doc-test --check-build --api-site \
  --ignore-dir=incubation --ignore-dir=openstack-compute-api-1.0

Change-Id: I6bad41543ed2269217a2ed36fcd947b83707daf2
Merge "Do not download RNG Schema, instead use local copies"
Merge "Add parameter --ignore-dir"