Adding generic trust creation and destruction methods

This change adds a minor refactor to the sahara.service.trusts module to
add generic creation and destruction methods as well as specific methods
used during transient cluster management.

These changes are in support of greater trust usage within the Sahara
code base. Previously only transient cluster management used trusts and
the creation/desutruction methods were customized to that end.

Changes
* renamed create_trust to create_trust_for_cluster
* renamed delete_trust to delete_trust_from_cluster
* added create_trust
* added delete_trust
* changed sahara.service.ops to use new _cluster specific trust methods
* added logging messages to trust creation and deletion
* changed delete trust to throw a DeletionFailed exception
* added CreationFailed exception

Partial-implements: blueprint edp-swift-trust-athentication
Change-Id: I61087ff76e8237e5a7999fc509b60260d4529476
Merge "Adding generic trust creation and destruction methods"