Allow %h placeholder in discoveryAgentUrl

[rhuss]

A placeholder given in the discoveryAgentUrl like %h should be replaced with an automatically detected hostname or IP address which is not the loopback host.

See also the discussion in hawtio/hawtio#1158.

[rhuss]

The following placeholder should be supported

• $host : Host name (if possible), otherwise address
• $ip : IP Address
• ${prop:foo} : System property foo
• ${env:FOO} : Environment variable FOO