Configuration might be unmarshal and altered before usage into templating

[rbeuque74]

Describe the bug
When using configuration in templating, if the value fetch from configstore is a raw value (such as a SSH key),
utask engine try to unmarshal it with yaml

utask/engine/engine.go

Line 89 in 53a7164

err := yaml.Unmarshal([]byte(*v), &i, func(dec *json.Decoder) *json.Decoder {

In the case of a SSH key, the yaml.Unmarshal succeeds, but transform all \n into spaces.
The SSH key used in a utask template, given to the SSH plugin, is not usable.

Proposed fix
I guess we should add a new templating store, named configraw, and put the raw value there, without any yaml.Unmarshal.
In that way, if user want to use a raw value such as a SSH key, it can come from this store.

utask version impacted by the bug
All

cc @Alkorin