Add setting to throw exception in non-interactive mode if some params are missing

[fduch]

This PR covers #63 and provides new exception-if-missing configuration option.

By default the behavior of script handler is the same as current. But when exception-if-missing option is set to true ParameterHandler will throw an exception when some parameters are missing. This is very useful during build/deploy process (executed for example on build/ci servers) when it is needed only to validate parameters.yml file before installing the application and abort the process when something went wrong, but not to change it's original content in any way.

This behavior affects only non-interactive mode.

[fduch]

@stof any chance that this PR would be reviewed?

[ZeeCoder]

+1 for such a functionality

[fduch]

@ZeeCoder since @stof didn't give any feedback to this PR i use exception-if-missing branch in my fork of the official repository without any issues for almost a year.. :)

[ZeeCoder]

@fduch Nice.
Actually I don't even get how others do deployment without something like this.
Well, I solved it a way too, but I still don't get how it doesn't seem to be a big issue for others.

I think this feature is needed. 👍

Any opinion on the matter @stof ?

[andrerom]

+1 , but:

Actually I don't even get how others do deployment without something like this.

Manage fine without this making sure params have sensible defaults.
A bigger problem is how verbose install is starting to become by relying only on ParameterHandler for human installer input.

[alanpoulain]

+1 Merging this PR would solve a lot of issues we have with our continuous integration.

[Restless-ET]

👍

[JonasHaouzi]

@stof Can we help you reviewing this PR? It'd be indeed really helpful. Thanks.

[sandermarechal]

Any progress on this? @stof can this be added instead of #107? This is something I could really use in continuous deployment.