Allow to mark internal network as external

[Michkov]

With new enviroment variable LINODE_EXTERNAL_SUBNET is possible to set internal network subnet to be used as external network.

Useful for running on testing Linode cloud instances which are providing external IP addresses in internal network.

General:

• Have you removed all sensitive information, including but not limited to access keys and passwords?
• Have you checked to ensure there aren't other open or closed Pull Requests for the same bug/feature/question?

Pull Request Guidelines:

1. Does your submission pass tests?
2. Have you added tests?
3. Are you addressing a single feature in this PR?
4. Are your commits atomic, addressing one change per commit?
5. Are you following the conventions of the language?
6. Have you saved your large formatting changes for a different PR, so we can focus on your work?
7. Have you explained your rationale for why this feature is needed?
8. Have you linked your PR to an open issue

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 55.48%. Comparing base (76dea77) to head (fab127d).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
cloud/linode/node_controller.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #235      +/-   ##
==========================================
+ Coverage   55.39%   55.48%   +0.09%     
==========================================
  Files          12       12              
  Lines        1883     1887       +4     
==========================================
+ Hits         1043     1047       +4     
  Misses        696      696              
  Partials      144      144              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[rahulait]

lgtm, haven't run and tested the code, but otherwise looks good to me.