fix: missing location config via node name and server id

[lukasmetzner]

When merging our two binaries into one, the option to configure
the location on startup via KUBE_NODE_NAME or HCLOUD_SERVER_ID was not implemented.

Missing configuration options in docs:

csi-driver/docs/kubernetes/README.md

Lines 154 to 155 in 637e45c

	2. The location is derived by querying a server specified by the `HCLOUD_SERVER_ID` variable.
	3. If neither of the above is set, the `KUBE_NODE_NAME` environment variable defaults to the name of the node where the CSI controller is scheduled. This node name is then used to query the Hetzner API for a matching server and its location.

Fixes #1149

[codecov]

Codecov Report

❌ Patch coverage is 37.50000% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.64%. Comparing base (8048835) to head (3ea107e).

Files with missing lines	Patch %	Lines
cmd/main.go	25.00%	6 Missing and 3 partials ⚠️
internal/app/app.go	50.00%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1158       +/-   ##
===========================================
+ Coverage   37.62%   71.64%   +34.01%     
===========================================
  Files          17       17               
  Lines        1483     1481        -2     
===========================================
+ Hits          558     1061      +503     
+ Misses        905      336      -569     
- Partials       20       84       +64     

Flag	Coverage Δ
e2e-controller	58.60% <37.50%> (?)
e2e-node	58.60% <37.50%> (?)

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[jooola]

Is there an easy way to test this code?

[lukasmetzner]

Is there an easy way to test this code?

I could think of an integration test where we spin up a test server and a fake metadata server, to test all four paths of GetServerLocation.