nm_pacrunner_manager_remove() required a "tag" argument. It was a
bug for callers trying to remove a configuration for a non-existing
tag.

That effectively means, the caller must keep track of whether a certain
"tag" is pending. The caller also must remember the tag -- a tag that he
must choose uniquely in the first place.

Turn that around and have nm_pacrunner_manager_send() return a (non
NULL) call-id. This call-id may later be used to remove the
configuration.

Apparently, previously the tracking of the "tag" was not always correct
and we hit the assertion in nm_pacrunner_manager_remove().

https://bugzilla.redhat.com/show_bug.cgi?id=1444374
(cherry picked from commit b04a9c90)