govc: snapshot.tree json output to stdout

[jlehtnie]

golang builtin println is meant for debugging only and prints always to stderr

Description

Print json to stdout instead of stderr.

Closes: #3392

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
• Build related change

How Has This Been Tested?

Run command:
govc snapshot.tree -json -vm $vm 2> /dev/null

Obverve json appearing in the terminal.

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

[vmwclabot]

@jlehtnie, you must sign our contributor license agreement before your changes are merged. Click here to sign the agreement. If you are a VMware employee, read this for further instruction.

[vmwclabot]

@jlehtnie, we have received your signed contributor license agreement. The review is usually completed within a week, but may take longer under certain circumstances. Another comment will be added to the pull request to notify you when the merge can proceed.

[dougm]

Thanks @jlehtnie

[vmwclabot]

@jlehtnie, VMware has approved your signed contributor license agreement.