WIP: Split config PoC

[JoshVanL]

What this PR does / why we need it:
Adds a prefixes flag to set prefixes of files that should be included in merging config files.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #278

NONE

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[jetstack-bot]

@JoshVanL: PR needs rebase.

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[jetstack-bot]

@JoshVanL: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
tarmak-puppet-module-tarmak-acceptance-1-14-centos	9e55366	link	/test puppet-tarmak-acceptance-centos v1.14

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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