CertUpdate: make it easy to invoke from other programs
The guts of ipa-certupdate are useful to execute as part of other
programs (e.g. as a first step of ipa-ca-install). Refactor
ipa_certupdate.CertUpdate to make it easy to do that. In
particular, make it possible to use an already-initialised API
object.
Part of: https://pagure.io/freeipa/issue/6577
Reviewed-By: Rob Crittenden <rcritten@redhat.com>