Omit error message if 100-crio-bridge.conf has already been disabled

[tstromberg]

Fixes #9481

If you restart a flannel cluster, you get an annoying error message displayed to stderr:

E1017 11:05:17.891097   21588 flannel.go:656] unable to disable /etc/cni/net.d/100-crio-bridge.conf: sudo mv /etc/cni/net.d/100-crio-bridge.conf \etc\cni\net.d\DISABLED-100-crio-bridge.conf: Process exited with status 1
stdout:

stderr:
mv: cannot stat '/etc/cni/net.d/100-crio-bridge.conf': No such file or directory

This PR silences this spurious error message by checking first if the file exists. Tested using:

1. minikube start --network-plugin=cni --cni=flannel --driver=docker
2. minikube stop
3. minikube start

This PR is not a behavioral change other than removing the stderr message.

[tstromberg]

/ok-to-test

[codecov-io]

Codecov Report

Merging #9505 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9505      +/-   ##
==========================================
- Coverage   29.08%   29.05%   -0.03%     
==========================================
  Files         171      171              
  Lines       10443    10443              
==========================================
- Hits         3037     3034       -3     
- Misses       6983     6987       +4     
+ Partials      423      422       -1     

Impacted Files	Coverage Δ
cmd/minikube/cmd/start_flags.go	47.16% <0.00%> (-0.60%)	⬇️
cmd/minikube/cmd/start.go	9.38% <0.00%> (-0.15%)	⬇️

[minikube-pr-bot]

kvm2 Driver
error collecting results for kvm2 driver: timing run 0 with Minikube (PR 9505): timing cmd: [/home/performance-monitor/.minikube/minikube-binaries/9505/minikube start --driver=kvm2]: starting cmd: fork/exec /home/performance-monitor/.minikube/minikube-binaries/9505/minikube: exec format error
docker Driver
error collecting results for docker driver: timing run 0 with Minikube (PR 9505): timing cmd: [/home/performance-monitor/.minikube/minikube-binaries/9505/minikube start --driver=docker]: starting cmd: fork/exec /home/performance-monitor/.minikube/minikube-binaries/9505/minikube: exec format error

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: medyagh, tstromberg

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [medyagh,tstromberg]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment