This patch makes Flexisip to use its hostname or cluster name while
generating 'Recorded-Route' and 'Path' headers.

The hostname is found out while parsing the first declared SIP URI
in 'transports' setting whereas the cluster domain must be specified by
the administrator in 'cluster' section.

'Record-Route' headers will always be generated from the hostname
whereas cluster name will be used as a priority for generating
Path headers using the hostname as fallback if cluster mode is not enabled
or no cluster name has been declared.

These modifications should improve Flexisip's resilience against NAT
routers and is necessary for setting a Flexisip push gateway cluster up.