Add ssh client module code

Add ssh client module code

Add ssh client module which support ssh key mode and interactive mode.
with --sshkey option you can provide the ssh key to be installed

>why not call it "ssh" module?
ssh-client is better, maybe future there will be ssh-server come in.
In debian these are also two different packages.

Usage:
1. sshkey mode:
transfer your public key to remote machine with ssh-copy-id or do it mannaully
example of options:
./dracut -l -H -a ssh-client --sshkey /root/.ssh/id_rsa i.img
2. interactive mode:
need use --ctty option, ie.:
./dracut -l -H -a ssh-client --ctty i.img

[v2 changes]:
per wangcong: add patch description about module name
add help line in usage()
remove useless comment

Signed-off-by: Dave Young <dyoung@redhat.com>