MEDIUM: checks: Support expression to set the port

MEDIUM: checks: Support expression to set the port

Since we have a session attached to tcp-check healthchecks, It is possible use
sample expression and variables. In addition, it is possible to add tcp-check
set-var rules to define custom variables. So, now, a sample expression can be
used to define the port to use to establish a connection for a tcp-check connect
rule. For instance:

    tcp-check set-var(check.port) int(8888)
    tcp-check connect port var(check.port)