JENKINS-18461 Allow the user to not allocate a public ip address for a slave in a cloudstack cloud

This patch adds an advanced option where the user can select to assign a public ip address to a jenkins slave. This option is useful when cloudstack is running in advanced networking mode and the jenkins server is located in the same isolated network or the isolated network is bridged to a physical vlan. In those cases the slave will use the internal IP address of the slave to connect, this saves a public IPv4 address.