Add option to load perf dash metrics from local artifacts directory

[Jont828]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This adds an additional metrics bucket type called local which allows users to load perf dash metrics from a local directory, specifically the output from running clusterloader2 with the --reportdir flag set. This is primarily used for development purposes so perf dash is usable with job data that is not hosted on prow.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

[mboersma]

/retest

/home/prow/go/pkg/mod/k8s.io/[email protected]/plugin/pkg/client/auth/exec/exec.go:35:2: golang.org/x/[email protected]: read "https://proxy.golang.org/golang.org/x/crypto/@v/v0.0.0-20190308221718-c2843e01d9a2.zip": stream error: stream ID 31; INTERNAL_ERROR; received from peer

Seems like a network flake? Edit: yep.

[mboersma]

/hold

This still needs the commits squashed and may be WIP.

[Jont828]

/hold cancel

[Jont828]

@mboersma Squashed and should be good to go.

[marseel]

looks good, thanks for the contribution. I guess that code-workspace file was accidentally added?

[marseel]

Thank you! /lgtm /approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Jont828, marseel, mboersma

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