Allow supervisor hostname override

[100mik]

What this PR does / why we need it:
Allows folks to configure an override for the hardcoded hostname, this enables cases like allowing CPI to speak with the API server through a VCF A proxy.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #1709

Special notes for your reviewer:

Release note:

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[DanielXiao]

This looks like a back door, I prefer to add hostName and port in CPI Config file. When they are empty, use SupervisorAPIServerFQDN as default.