nsenter command path should be configurable

[mgrzybek]

Some commands can be given using arguments. However, nsenter is not. The hard-coded path is /usr/bin/nsenter, however sometimes is available at /bin/nsenter.

In the current stable version:

kured/cmd/kured/main.go

Line 266 in e9a5d3b

cmd := []string{"/usr/bin/nsenter", fmt.Sprintf("-m/proc/%d/ns/mnt", pid), "--"}

In main:

kured/pkg/checkers/checker.go

Line 97 in 02e36de

cmd = append(cmd, "/usr/bin/nsenter", fmt.Sprintf("-m/proc/%d/ns/mnt", pid), "--")

We should have a new argument called --nsenter-path so be able to configure it.