feature: lazy connect to syslog server

[fuweid]

Ⅰ. Describe what this PR did

The syslog driver will try to connect syslog server. However, the syslog
server might not start. We cannot guarantee the syslog server must
start before container.

For this case, we can postpone the connection initialize. syslog driver
only does validation during initialize. When caller try to write data,
it will connect to server.

Ⅱ. Does this pull request fix one issue?

NONE

Ⅲ. Describe how you did it

Reimplement the syslog functionality to allow lazy connect

Ⅳ. Describe how to verify it

start a container

sudo pouch run --log-driver syslog --log-opt syslog-address=tcp://127.0.0.1:515 busybox sh -c "while true; do date; sleep 1; done"

wait for it and see the output on the shell.

$ sudo nc -l -p 515
<30>Aug  9 16:35:44 6abb5861fe82[18670]: Thu Aug  9 08:35:44 UTC 2018
<30>Aug  9 16:35:45 6abb5861fe82[18670]: Thu Aug  9 08:35:45 UTC 2018
<30>Aug  9 16:35:46 6abb5861fe82[18670]: Thu Aug  9 08:35:46 UTC 2018
<30>Aug  9 16:35:47 6abb5861fe82[18670]: Thu Aug  9 08:35:47 UTC 2018

open the nc to receive the message.

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #2068 into master will increase coverage by 0.33%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #2068      +/-   ##
==========================================
+ Coverage   63.93%   64.26%   +0.33%     
==========================================
  Files         206      208       +2     
  Lines       15762    15860      +98     
==========================================
+ Hits        10077    10192     +115     
+ Misses       4435     4406      -29     
- Partials     1250     1262      +12

Flag	Coverage Δ
#criv1alpha1test	33.06% <0%> (-0.21%)	⬇️
#criv1alpha2test	33.69% <0%> (-0.27%)	⬇️
#integrationtest	38.54% <51.78%> (+0.65%)	⬆️
#unittest	24.13% <64.28%> (+0.36%)	⬆️

Impacted Files	Coverage Δ
daemon/logger/syslog/conn.go	30% <30%> (ø)
daemon/logger/syslog/dialer.go	68.57% <68.57%> (ø)
daemon/logger/syslog/syslog.go	74.71% <87.71%> (+49.71%)	⬆️
ctrd/image.go	79.31% <0%> (-1.98%)	⬇️
daemon/mgr/container.go	55.31% <0%> (-0.22%)	⬇️
daemon/mgr/container_validation.go	33.04% <0%> (+2.6%)	⬆️
daemon/logger/syslog/validate.go	55.78% <0%> (+3.15%)	⬆️
daemon/mgr/container_logger.go	92.3% <0%> (+7.69%)	⬆️
... and 3 more

[allencloud]

LGTM