renaming variable to avoid shadowing
skipping undefined keys when filtering resources

When filter_resources is unable to perform a substitution due to neither
finding a suitable project attribute nor a project property,
it will leave the substitution untouched.
This allows to use filter_resources on files which also perform
string.Template substitutions themselves.
refactor rename variables for clarity
port touched tests to use @aelgru's fluentmock
emitting warning when substitution is not possible
use fluentmock anyway, error message will improve
Merge pull request #108 from pybuilder/fix/filter_resources_skip_unknown_keys

Fix/filter resources skip unknown keys