ID attribute in EntityDescriptor is unstable

[dnmvisser]

The EntityDescriptor's ID element in the metadata is generated here:

pysaml2/src/saml2/metadata.py

Line 824 in 9e597e1

ident = sid()

This value has some randomness:

pysaml2/src/saml2/s_utils.py

Line 189 in 9e597e1

def sid():

This results in different metadata files every time they are generated, even though nothing has actually changed. This prevents caching mechanism from working (checksums, HTTP Etag, etc).

A similar issue existed in SimpleSAMLphp, which was fixed by constructing the ID from the content: simplesamlphp/simplesamlphp@4fdb5a3

I tried that approach here: master...dnmvisser:pysaml2:dv_stablise_ids1 and that appears to work.

Since there is no requirement for ID to be random, I think this would be a nice improvement?