datapaths: Fix port name generation
datapaths: Fix port name generation

Port names generated by _generate_port() were longer than allowed.
This changes how they're generated so that they're still useful
but a bit shorter.

Thanks to damonmeen for the bug report.

Closes #65

VamsikrishnaNallabothu/pox,PrincetonUniversity/pox,VamsikrishnaNallabothu/pox,kulawczukmarcin/mypox,kulawczukmarcin/mypox,jacobq/csci5221-viro-project,kulawczukmarcin/mypox,PrincetonUniversity/pox,diogommartins/pox,andiwundsam/_of_normalize,kavitshah8/SDNDeveloper,carlye566/IoT-POX,kulawczukmarcin/mypox,jacobq/csci5221-viro-project,kpengboy/pox-exercise,adusia/pox,waltznetworks/pox,kpengboy/pox-exercise,xAKLx/pox,adusia/pox,denovogroup/pox,carlye566/IoT-POX,waltznetworks/pox,kavitshah8/SDNDeveloper,jacobq/csci5221-viro-project,kulawczukmarcin/mypox,kpengboy/pox-exercise,PrincetonUniversity/pox,andiwundsam/_of_normalize,chenyuntc/pox,denovogroup/pox,diogommartins/pox,PrincetonUniversity/pox,diogommartins/pox,kpengboy/pox-exercise,chenyuntc/pox,pthien92/sdn,MurphyMc/pox,MurphyMc/pox,pthien92/sdn,denovogroup/pox,denovogroup/pox,MurphyMc/pox,adusia/pox,waltznetworks/pox,xAKLx/pox,VamsikrishnaNallabothu/pox,pthien92/sdn,xAKLx/pox,noxrepo/pox,carlye566/IoT-POX,kavitshah8/SDNDeveloper,MurphyMc/pox,VamsikrishnaNallabothu/pox,waltznetworks/pox,pthien92/sdn,adusia/pox,diogommartins/pox,waltznetworks/pox,carlye566/IoT-POX,andiwundsam/_of_normalize,noxrepo/pox,chenyuntc/pox,andiwundsam/_of_normalize,xAKLx/pox,VamsikrishnaNallabothu/pox,diogommartins/pox,kpengboy/pox-exercise,denovogroup/pox,MurphyMc/pox,carlye566/IoT-POX,xAKLx/pox,jacobq/csci5221-viro-project,noxrepo/pox,PrincetonUniversity/pox,kavitshah8/SDNDeveloper,chenyuntc/pox,pthien92/sdn,adusia/pox,chenyuntc/pox,jacobq/csci5221-viro-project,noxrepo/pox