Implement `print` method for `server_list_settings`

Right now, when you run `puppet config print server_list`, you get a gross looking array. This should print what you see in `puppet.conf`, a comma separated list of servers. Not the strange ruby array we get now

[1] Melissa@bubba:/Users/Melissa/puppet:(5.5.x)$ be ./bin/puppet config set server_list server_list_1,server_list_2

[0] Melissa@bubba:/Users/Melissa/puppet:(5.5.x)$ be ./bin/puppet config print server_list

[["server_list_1"], ["server_list_2"]]

[0] Melissa@bubba:/Users/Melissa/puppet:(5.5.x)$ cat ~/.puppetlabs/etc/puppet/puppet.conf

[main]

server = server1

server_list = server_list_1,server_list_2

It looks like this is a more general problem for settings that derive from `:array`.

All of this is from josh's comment:
https://github.com/puppetlabs/puppet/pull/7384#issuecomment-462976534

to quote him,

I'd expect the `ArraySetting` to implement `print` to join using comma:

def print(value)

vals = super(value)

vals.join(',')

end

And for `ServerListSetting` to override it so that each host and optional port are first joined with ':', and then that result is comma-joined:

def print(value)

vals = munge(value)

super(vals.map { |v| v.join(':') })

end