Cybersource Rest: Stored Credential refactor

[aenand]

COMP-78

Refactors the stored credential support for the Cybersource Rest gateway to be in-line with their documentation. Also repairs test suite for this gateway by eliminating certain tests and fixing others.

Test summary: Local: 5838 tests, 79156 assertions, 2 failures, 23 errors, 0 pendings, 0 omissions, 0 notifications 99.5718% passed Unit: 30 tests, 144 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications 100% passed Remote: 43 tests, 143 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications 100% passed