1035115: Update product id certs

Add support for updating the install product id
certs to newer versions.

productid.py now checks available versions against
installed versions, and compares the certs with
ComparableProductCert and ComparableProduct classes.

Versions newer than installed systems are updated.

Add a "pre_product_id_update" and "post_product_id_update"
plugin slots, and corresponding plugin conduit.

Split installing of new product certs, and updating
old ones so we can have seperate plugins.
wip: add comparable Productid class
Merge pull request #875 from candlepin/alikins/productid_update

1035115: Update product id certs