Add Linux VM kind

[jmussmann]

This PR add support for a vrnlab linux VM kind.

The corresponding PR in the vrnetlab repository is "Add Ubuntu Support #183" for ubuntu.

The code would also support other linux distribution. This is why the kind is called vr_linux and not vr_ubuntu.

[codecov]

Codecov Report

Attention: Patch coverage is 6.25000% with 30 lines in your changes are missing coverage. Please review.

Project coverage is 53.46%. Comparing base (cf84f5f) to head (6fcac59).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1976      +/-   ##
==========================================
- Coverage   53.64%   53.46%   -0.18%     
==========================================
  Files         160      161       +1     
  Lines       11538    11570      +32     
==========================================
- Hits         6189     6186       -3     
- Misses       4485     4518      +33     
- Partials      864      866       +2     

Files	Coverage Δ
clab/register.go	100.00% <100.00%> (ø)
nodes/vr_linux/vr-linux.go	3.22% <3.22%> (ø)

... and 1 file with indirect coverage changes