Use a container ClientContainer for meta interface{} to extend

[Didainius]

This code contains many search/replace things, but core change is in provider.go

The root thing is that the meta interface{} attribute will be type ClientContainer instead of *VCDClient.
Instead it is:

type ClientContainer struct {
	tmClient *VCDClient
        futureClient *OtherClient <------ not existing now
}

Such approach allows us to expand the meta argument that is used in all SDK method with additional structures that might be required.

The difference for consumption is that it becomes:

vcfaClient := meta.(ClientContainer).tmClient

instead of:

vcdClient := meta.(*VCDClient)

in each SDK consuming

Tests passed without any panics which should confirm that there are no missing replacements.

[adambarreiro]

LGTM

[lvirbalas]

Looks great, just PR still mentions "meta" :)

[Didainius]

Looks great, just PR still mentions "meta" :)

The parts of meta interface{} will stay, because this is the form which Terraform SDK uses to pass ClientContainer around all the CRUD functions