Add NamespaceDB support to pkg/vsphere/extraconfig

[hickeng]

Story
As a VIC developer I want to be able to easily serialize/deserialize configurations via a VM namespace

Details
In-guest support depends on #4061
Port layer side depends on #4065 for the types so we can construct a DataSpec array for the VMODL operation

Testing
Portlayer side: Unit tests for the generation of the DataSpec array
Guest side: very basic tool as per rpctool - see namespacetool