examples: add toolbox Client.Run

[dougm]

Description

Add simulator.RunContainer method, simplifies Go tests that depend on container backed simulated VMs.
Move some initialization from govc to a toolbox.NewClient method that can be reused.

Closes: #2458

Type of change

Please mark options that are relevant:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Existing guest.bats tests
• The new example is run as a test itself

Checklist:

• My code follows the CONTRIBUTION guidelines of
  this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

[embano1]

Nice work, really useful!

[embano1]

Currently we don't highlight examples: in the CHANGELOG. Let me know if I should add a highlight section there.

[dougm]

@embano1 yes, thanks, it'd be great to highlight examples: in the CHANGELOG

[embano1]

LGTM