Deprecate cloud prop for Global Nav

[interactivellama]

We should not promote the use of this. I think SLDS has decided that they are not administrating these themes and it's up to each cloud to overwrite styles/implement.

checkprops should be added to detect use of cloud prop and tell devs.

[interactivellama]

This just adds a class, whether SLDS supports it or not is up to SLDS. Right now, .slds-context-bar--theme-marketing is the only one in SLDS 2.x