(PUP-2349) Force mode property to be a string

The initial commit for this ticket missed the case where
the mode value isn't explicitly specified but rather comes from
source permissions of a file resource, e.g. as happens during
pluginsync.

This change ensures that mode from a source permission is
converted to an octal string if it's a Numeric.
(PUP-2349) Specify mode as an octal string consistently in spec manifest snippets

In the puppet 4.0 parser, the file resource's mode property will be
required to be a string containing an octal (or symbolic) representation
of mode.

For consistency then, this commit changes all cases where mode was
specified as a Numeric to be a string containing an octal representation
of mode (always prefaced with a 0, again for clarity and consistency).
Merge pull request #3022 from kylog/issue/pup-2349/cleanup-warning-during-pluginsync

(PUP-2349) cleanup warning during pluginsync.