azk agent start: Fix configure errors

[saitodisse]

This closes #444

Now its is not necessary to catch agent.configure errors when agent is starting. This way the process will correctly stop when some errors occours.

Because of https://github.com/azukiapp/azk/blob/07ee52476c/src%2Futils%2Fpromises.js#L4-L11 no more Unhandled Rejection will be showed to final users.

To hold 80 port and test we can use socat:

sudo apt-get install socat
sudo socat TCP4-LISTEN:80,fork EXEC:"echo `pwd`"

To hold 80 port with azk container we can kill -9 the process while azk agent is up:

kill -9 $(pgrep azk);