This patch set changes Pegleg in two similar ways:
1) Ignore certain types of files altogether:
- those located in hidden folders
- those prefixed with "." (files like .zuul.yaml)
2) Only read Deckhand-formatted documents for lint/collect/etc.
commands as Pegleg need not consider other types of documents
(it separately reads the site-definition.yaml for internal
processing still).
The tools/ subfolder is also ignored as it can contain
.yaml files which are not Deckhand-formatted documents,
so need not be processed by pegleg.engine.
Change-Id: I8996b5d430cf893122af648ef8e5805b36c1bfd9