Make RequestContext use auth_token (not auth_tok)

The RequestContext class from oslo-incubator uses 'auth_token'.
Glance's RequestContext uses 'auth_tok' -- which is inconsistent.

glance_store currently uses the oslo 'auth_token'.

We should not assume a 1-1 mapping from the glance_store to glance,
nor should we have glance_store support both 'auth_token' and
'auth_tok'. Therefore Glance should be updated to use 'auth_token'.

This change was generated automatically with:

$ for file in `find glance -name '*\.py' -exec grep -wl auth_tok "{}" \;` ; \
  do sed --in-place 's/\<auth_tok\>/auth_token/g' $file ; done

In addition, the set_auth_token function was removed. This had been used
by Nova to work around the auth_tok <-> auth_token inconsistency, but
was removed in mid-2012 as part of the move to python-glanceclient so
is no longer needed.

In conjuction with other changes this fixes image upload when
using the multi-tenant Swift store.

Change-Id: Ic8a5f44088990fd8f6290a5622b823f59ef365fc
Partial-bug: #1385213
Replace custom lazy loading by stevedore

Now we have own implementation for loading
extensions. stevedore avoids creating yet
another extension mechanism by building on
top of setuptools entry points.

Change-Id: I7fcb981b58b20fb49d10eea3d477f962e4bf3565
Merge "Make RequestContext use auth_token (not auth_tok)"
Merge "Replace custom lazy loading by stevedore"