Add option for simultaneous suite builds, adjust outpath for local builds

[jonathanspw]

Adds the -j argument to the command line to build multiple suites simultaneously. The default is 1 so it is in line with previous functionality.

Use absolute paths for outpath to make it more compatible with non-docker uses. Existing functionality/defaults are maintained when the -o option is not passed.

[google-cla]

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[google-oss-prow]

Hi @jonathanspw. Thanks for your PR.

I'm waiting for a GoogleCloudPlatform member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[drewhli]

/ok-to-test