Make vic-machine support taking datastore path for appliance and bootstrap isos

[hmahmood]

Currently we upload the bootstrap and appliance iso's every time we create a VCH for the integration tests (each time we run a .robot file). We really just need to reference the already built iso's every time we create a VCH.

vic-machine already has an option to specify appliance and bootstrap iso file paths, --appliance-iso and --bootstrap-iso, and we can make those options use datastore paths instead; vic-machine would simply not upload any iso's, and configure the VCH properly to use the iso's specified by the datastore path.

This could also be useful in a customer environment where the customer may want to deploy several VCH's but have them reuse the isos from one location.

[mdubya66]

Putting this in Icebox for now.

[zjs]

Should be considered in conjunction with shared image stores.