Add support for gov-cloud (fixes #96)

[pendor]

This adds a new preference to select which cloud environment to target.
Endpoint URLs for additonal clouds are taking from MS documentation.
All calls to cloud API's now check the cloud preference and select
the correct endpoint for the current environment.

[chris1984]

Starting to Test

[chris1984]

ACK, Works great @pendor

Was able to connect Foreman to the GOV Cloud Compute Resource:

https://1drv.ms/u/s!AmcYeL73Lgj6hltDyFlGbzsujPKv?e=7chTbr

Validated that it would not take a normal Azure account by putting my normal one in it's place

https://1drv.ms/u/s!AmcYeL73Lgj6hybS4EV2s_Q0SDVr?e=WVL4Gv

Was able to create a VM, screenshot showing it building

https://1drv.ms/u/s!AmcYeL73Lgj6hyeO5lpT8tD9vEgx?e=k8FEsf

Finished product in Foreman

https://1drv.ms/u/s!AmcYeL73Lgj6hyjqU-gAFqyemETS?e=Px3j01

Able to list VM's

https://1drv.ms/u/s!AmcYeL73Lgj6hySwqSjAtlG1PWGO?e=qdAGPg

Also tested that it didn't break anything with a normal Azure account and was able to list/build a vm, and delete

@apuntamb can this get a merge, will get my docs pr in and then we can do a new version release.

I am going to demo this Thursday at the Foreman community demo.

Also going to write up the steps needed to go from installing the plugin to getting it working since, I noticed in our upstream/downstream docs we don't talk about that much.

[apuntamb]

Thanks for the feature @pendor and @chris1984! 👍
Merged!