Support arbitrary YAML as parameter values

[nfnt]

What this PR does / why we need it:
This allow templates to access list or dictionary items if parameter values are provided as valid YAML. Because a simple string is also valid YAML, it is still compatible with regular string values.

Currently WIP, I plan to create a KEP around this as well which will provide more background and motivation.

Fixes #1349

[nfnt]

It's not that easy :(
Template functions like eq now fail because it's no longer a string-string comparison.

[porridge]

Also, I suspect passing valid non-trivial YAML as value of the -p command line option will not exactly be easy...

[nfnt]

@porridge, I agree, my main focus wasn't -p just yet.

eq fails because by treating everything as YAML, unmarshalling converts some types. E.g., the input string "true" is converted to a boolean. The eq comparison expects to compare a string with a string and fails because it's now comparing a bool with a string.

[nfnt]

Closing, as this is superseded by KEP-24.