NBCTL: Fix missing `execute()` when adding ACLs

[mkalcok]

Missing execute() calls in OvnNbctl.acl_add() method resulted in ACLs never getting created in OVN Northbound database.

Testing/Verification:

• Run ocp-20-np-multitenant test scenario which utilises ACLs
• Log into OVN ovn-central container on OVN-CENTRAL host
• Dump ACL table
  • ovsdb-client dump unix:/var/run/ovn/ovnnb_db.sock OVN_Northbound ACL

Before this change the table would be empty during and after the test scenario.