To prepare for implement general task queue mode to expirer,
this patch splits expirer's method into smaller ones and parametrize task
account. This change will make expirer's general task queue patch [1] more
simple.
This patch has following approaches:
1: Split methods into smaller ones
2: Parameterize task account name to adapt many task accounts
in general task queue
3: Include task account names in log messages
4: Skip task account when the account has no task containers
[1]: https://review.openstack.org/#/c/517389/
Change-Id: I907612f7c258495e9ccc53c1d57de4791b3e7ab7
1fadffeae0