Populate srl mgmt0 config with runtime mgmt bridge mtu

[steiler]

Implementing #1655

[hellt]

@steiler check this
unfortunately we can't use link mtu lower than 1500
to make it work we need to use ip mtu.
This is what I was alluding to in the original issue.

take a look at this change in 257917b
I don't quite like it, but maybe for the time being it will do

[codecov]

Codecov Report

Merging #1658 (257917b) into main (670d658) will increase coverage by 0.02%.
The diff coverage is 69.44%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1658      +/-   ##
==========================================
+ Coverage   50.96%   50.99%   +0.02%     
==========================================
  Files         136      136              
  Lines       13188    13202      +14     
==========================================
+ Hits         6721     6732      +11     
- Misses       5726     5727       +1     
- Partials      741      743       +2     

Files	Coverage Δ
types/types.go	60.00% <ø> (ø)
runtime/podman/util.go	77.77% <0.00%> (ø)
nodes/srl/srl.go	57.19% <37.50%> (-0.01%)	⬇️
runtime/docker/docker.go	64.59% <73.46%> (-0.08%)	⬇️

... and 1 file with indirect coverage changes

[steiler]

lgtm