Add a NotLockingAccounts credentials provider

This credentials provider does not do any locking, so it
does not suffer from the lack of credentials cleanup which
affects the regular locking Accounts class.

Using thie credentials provider allows to keep it possible
to run tempest with no tenant isolation, and in serial mode,
as it is done today by one of periodic jobs.

Partially-implements bp:test-accounts
Change-Id: I160d183a299d2e46d269428409af680c0fdb03b5
Merge "Add a NotLockingAccounts credentials provider"