The framework/ssh.go code was heavily used throughout the framework
and could be useful elsewhere but reusing those methods requires
importing all of the framework.
Extracting these methods to their own package for reuse.
Only a few methods had to be copied into this package from the
rest of the framework to avoid an import cycle.
This is part of the transition to using framework/log instead
of the Logf inside the framework package. This will help with
import size/cycles when importing the framework or subpackages.
The existence of /proc/net/nf_conntrack depends on the Linux kernel
config CONFIG_NF_CONNTRACK_PROCFS, and Ubuntu 16.04's one is disabled.
So the e2e test "should set TCP CLOSE_WAIT timeout" fails on that.
This adds the existence check of /proc/net/nf_conntrack for skipping
the test if not existing.
In addition, this makes IssueSSHCommandWithResult return Stderr in
the error message if err is nil to check "No such file or directory"
as nonexistence of /proc/net/nf_conntrack.
