Consistent spruce usage for terraform sub pipelines

[o-orand]

Expected behavior

As a template author, I want a consistent spruce usage in order to be able to use this feature.

Observed behavior

On terraform manifest generation SPRUCE_FILE_BASE_PATH is not set, so all paths used to reference a file a relative to /tmp/build/xxxxx instead of /tmp/build/xxxxx/credentials-resource/<%terraform_path%

Affected release

Reproduced on version 3.4.1

Traces and logs

processing terraform-tpl.tfvars.yml	
generating terraform.tfvars.yml	
2 error(s) detected:	
 - $.internal-ca-1: tried to read file ../../shared/certs/ca-1.crt: could not be read - open ../../shared/certs/ca-1.crt: no such file or directory
 - $.internal-ca-2: tried to read file ../../shared/certs/ca-2.crt: could not be read - open ../../shared/certs/ca-2.crt: no such file or directory

[gberche-orange]

similar to #259 , may need to see if common solution can be designed for both issues