Previously 'nmcli con modify' appended values for multi-value properties.
This commit makes 'nmcli con modify' overwrite the whole value. You can
choose appending values by prefixing the setting.property with '+' sign.
For simple (not container) properties the behaviour is the same both with
and without the '+', of course.

Synopsis:
  nmcli connection modify [+]<setting>.<property <value>

Example:
---> ipv4.dns = 1.2.3.4
$ nmcli connection modify my-em1 ipv4.dns 8.8.8.8
---> ipv4.dns = 8.8.8.8
$ nmcli connection modify my-em1 +ipv4.dns 8.8.4.4
---> ipv4.dns = 8.8.8.8 8.8.4.4

https://bugzilla.redhat.com/show_bug.cgi?id=1044027