storage_pool: Rework chap XML to mimic ceph

storage_pool: Rework chap XML to mimic ceph

The existing 'chap' XML logic was never used - just defined.  Rather than
try to insert a square peg into a round hole, blow it up and rewrite the
logic to follow the 'ceph' format.

Remove the former "chap.login" and "chap.passwd" fields and replace
with "chap.username" and "chap.secret" in _virStoragePoolAuthChap.
Adjust the virStoragePoolDefParseAuthChap() to process.

Change the rng file to describe the new layout

Update the formatstorage.html to describe the usage of the secret element
to mention that the secret type "iscsi" and "ceph" can be used
to storage pool too.

Update the formatsecret.html to include a reference to the storage pool

Update tests to handle the changes from 'login' and 'passwd' to 'username'
and '<secret>' format