Allow multiple providers for vmware transport

[bodepd]

Currently, the transport resource does not have any providers, it is intended as a simple way to specify data as resources.

This patch adds basic support for multiple providers so that credentials can be passed in ways other than as resources.

This is done by calling the getter methods for the properties that are used to retrieve credential information.

For my use case, it will allow me to store credentials in device.conf files (and to use my custom encryption/decryption mechanisms.

[bodepd]

let me test this a little more. It should have no adverse effects on the existing code (it just allows for specifying custom transport providers). To get a better feel for the changes, you may want to check without whitespace diffs.

[bodepd]

I have run some tests on this and feel pretty good about it

[nanliu]

This will allow credential to be provided from sources other than the manifests/catalog. Reading device.conf or encrypted credential will be possible using alternative providers.