Added centralized magic var replacer and support replacer for execs

[hellt]

fix #2393

it is immediately clear if doing a global replacement is possible due to the requirement to support nodeName variable that needs to have node name parsed first.

Instead, the node vars are being applied to the most wanted sections:

• binds
• execs
• startup-config

[codecov]

Codecov Report

Attention: Patch coverage is 85.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 51.92%. Comparing base (5132320) to head (99add4c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
clab/config.go	85.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2395      +/-   ##
==========================================
+ Coverage   51.90%   51.92%   +0.02%     
==========================================
  Files         172      172              
  Lines       17300    17316      +16     
==========================================
+ Hits         8979     8992      +13     
- Misses       7379     7381       +2     
- Partials      942      943       +1     

Files with missing lines	Coverage Δ
clab/config.go	68.16% <85.00%> (+0.46%)	⬆️