MINOR: agent-check: Be able to set absolute weight via an agent

MINOR: agent-check: Be able to set absolute weight via an agent

Historically, an agent-check program is only able to set a proportial weight
to the initial server's weight. However, it could be handy to also set an
absolute value. It is the purpose of this patch.

Instead of changing the current way to set a server's weight, a new
agent-check command is introduced. The string "weight:", followed by an
positive interger or a positive interger percentage, can now be used. If the
value ends with the '%' sign, then the new weight will be proportional to
the initially weight of the server. Otherwise, the value is considered as an
absolute weight and must be between 0 and 256.

This patch should fix the issue #360.