fails to properly handle multiple server definitions

[vidkun]

When configuring this role to use multiple time servers, it results in an incorrect server definition in the config file with an array defined. This causes chronyd to fail to start.
Providing the following vars:

chrony_server:
  - ntp-1.example.com prefer iburst
  - ntp-2.example.com prefer iburst

Results in the following line dropping into chrony.conf:

server [u'ntp-1.example.com prefer iburst', u'ntp-2.example.com prefer iburst']

It should instead result in:

server ntp-1.example.com prefer iburst
server ntp-2.example.com prefer iburst