BUG/MINOR: check: consitent way to set agentaddr

BUG/MINOR: check: consitent way to set agentaddr

small consistency problem with `addr` and `agent-addr` options:
for the both options, the last one parsed is always used to set the
agent-check addr.  Thus these two lines don't have the same behavior:

  server ... addr <addr1> agent-addr <addr2>
  server ... agent-addr <addr2> addr <addr1>

After this patch `agent-addr` will always be the priority option over
`addr`. It means we test the flag before setting agentaddr.
We also fix all the places where we did not set the flag to be coherent
everywhere.

I was not really able to determine where this issue is coming from. So
it is probable we may backport it to all stable version where the agent
is supported.

Signed-off-by: William Dauchy <wdauchy@gmail.com>