take metadata endpoint from config file

[IlyaMar]

Read metadata endpoint from /etc/google_oslogin.conf.
Use default 169.254.169.254 if config not found or not readable.
Read config once on .so load.

[google-oss-prow]

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[google-oss-prow]

Hi @IlyaMar. 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.

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[dorileo]

Hi @IlyaMar what are you trying to achieve wit this change? How would a user benefit from this feature? What would be the use case?

Thanks,
Leo

[IlyaMar]

Hi @IlyaMar what are you trying to achieve wit this change? How would a user benefit from this feature? What would be the use case?

Thanks, Leo

Hi Leo, sorry for a long delay. This is not for a user, rather for a developer to be able to test component against a mock endpoint that listens on localhost. Another possible case is migration to a new endpoint to avoid old endpoint shutdown and oslogin downtime.