cli: add 'nmcli connection clone' for cloning connections (bgo #757627)

cli: add 'nmcli connection clone' for cloning connections (bgo #757627)

Synopsis:
nmcli connection clone [--temporary] [id|uuid|path] <ID> <new name>

It copies the <ID> connection as <new name>. The command is very useful
if there is a connection, but another one is needed for a related
configuration. One can copy the existing profile and modify it for the
new situation.

For example:
$ nmcli con clone main-eth second-eth
$ nmcli con modify second-eth connection.interface-name em4

https://bugzilla.gnome.org/show_bug.cgi?id=757627