add in "secure" grains support

[verhulstm]

I want a grains dir in auth that contains a grains file.

The file should be the same format as sample-states/saltstack/grains/grains.

when rambo up is run i want to append the contents of auth/grians to the main saltstack/grains/grains file.

[verhulstm]

this appending needs to be done before line # 127 in https://github.com/terminal-labs/rambo/blob/master/rambo/Vagrantfile

[verhulstm]

Some saltstack users also add in home roled support for grains.d dirs.

Here they merge all files found in grains.d into one grains file. This does introduce grain overwriting/ordering issues. But if the grains namespace is managed well then these issues go away.

see: https://github.com/salt-formulas/salt-formula-salt/blob/master/salt/minion/grains.sls

Do we want this feature?

[verhulstm]

we should only append the extra grains to the grains file inside the rambo system. That way we reduce the chances of users accidentally committing secret grains to github.

[nixjdm]

I feel like we should hash out #218 first before starting this.