Multi-context - add/update/delete - l2xc instances - add 10, del 4, add new 4, del 10

HLD:
config1
configure 14 l2-pg interfaces
add 10 uni-directional x-connects (5 x 2 x-connects per pair of l2-pg interfaces)
test1
send L2 MAC frames between all pg-l2 interfaces
verify1
check l2 patch data by parsing output of bridge_domain_dump - not possible:
show l2patch
no l2patch entries
all packets received correctly in case of x-connected l2-pg interfaces
no packet received in case of not x-connected l2-pg interfaces

config2
delete 4 x-connects
test2
send L2 MAC frames between all pg-l2 interfaces
verify2
all packets received correctly in case of x-connected l2-pg interfaces
no packet received in case of not x-connected l2-pg interfaces

config3
add new 4 x-connects
test3
send L2 MAC frames between all pg-l2 interfaces
verify3
all packets received correctly in case of x-connected l2-pg interfaces
no packet received in case of not x-connected l2-pg interfaces

config4
delete 10 x-connects
test4
send L2 MAC frames between all pg-l2 interfaces (i.e. no x-connect created)
verify4
no packet received on all of l2-pg interfaces