Add more VM class methods, rename VDC to Region Quota, add more RegionQuotaStoragePolicy methods, fix NSX

[adambarreiro]

• Rename govcd.TmVdc to govcd.RegionQuota to use same naming as VCFA UI. types.TmVdc remains the same to respect the original name from the API type.

• Added VCDClient.AssignVmClassesToRegionQuota as a convenient method to assign VM classes without having to fetch the Region Quota.

• Rename govcd.TmVdcStoragePolicy to govcd.RegionQuotaStoragePolicy to use same naming as VCFA UI. types.VirtualDatacenterStoragePolicy remains the same to respect the original name from the API type.

Side changes:

• Removed NetworkProviderScope from NsxtManagerOpenApi as it is not recognized anymore.
• Added new NsxtManagerOpenApi properties

[lvirbalas]

Note: TM-specific features are in tm_*.go files (with the prefix). While the functions themselves only have Tm prefix where a distinction from a same-named VCD function is needed (so, mostly there is no prefix).