add validate commands for create and update json files for GH-153

[MerlinDMC]

To make @konobi happy and fix GH-153 this implements two validation commands:

vmadm validate create [-f <filename>]
vmadm validate update brand [-f <filename>]

I left the validation of receive JSON out because I think nobody will unbundle and edit those files.

I did change the main() callback because on failure flushLogs was called twice and sometimes the wrong one did call process.exit(0) which resulted in a zero exit code on failure.

[joshwilsdon]

resolved with commit 4c6d586, thanks for the help MerlinDMC