Support http health check request_path

[mDantas]

Hello!

I'm building here on the recent addition of support for http health checks. Namely, I'm adding a variable to specify the path at which to perform the http health check (as terraform-google-lb-http supports).

This seemed like the minimal change to achieve this functionality. I can see the argument for doing some refactoring and accepting a backend_params array instead as is the case with terraform-google-lb-http. Let me know how you'd like to proceed.

Thanks,
Matt

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[mDantas]

PS - I'm working on getting approval from my org to sign the CLA.

[omazin]

@aaron-lane think this can be closed since more parameters to health checks were added in #12

[morgante]

@mDantas Please reopen if your requirements weren't addressed.