Explicit parallelization support

[xlucas]

This declares the provider as capable of using parallelization by default. Without this, VMs are created sequentially and it can take a long time, for instance when extensively deploying from templates (see #32). In this case this is a huge time saver.

It can still be disabled with the --no-parallel option.

[rylarson]

Have you tested this fully? I did this a while back and I had problems bringing up many VM's in parallel. I spent some time chasing it down and I ran out of time to fix all of the locking required to make it work reliably.

[arhimondr]

I've tested it. Looks like it works well. Which kind of problems you had?

[rylarson]

It is possible that some unrelated changes in the plugin since I tried it out have fixed it since I tried it. I will give it another shot this week in my environment. We are running vSphere 5.5 and when I tried to bring up 100 VM's at once with the parallel provider, it would fail about 2-3 of them pretty consistently due to weird exceptions that seemed to be related to some rbvmomi objects not being thread safe.

[arhimondr]

It was looking stable for me because i have cluster with only 3 machines.
But recently i got similar error.