speed up operations by disabling known_hosts

Compare with this patch:

 $ time hivemind -H np-rcc55 libvirt.list
 [np-rcc55] Executing task 'list'
 [np-rcc55] run: virsh list
 [np-rcc55] virsh list success

 +----+--------------------------------------+-------------------+---------+
 | ID |                 UUID                 |        Name       |  State  |
 +----+--------------------------------------+-------------------+---------+
 | 2  | a6b092eb-58bf-41db-b751-6919ca27ed86 | instance-00008ddf | running |
 | 3  | dda6d997-7e35-4e70-8cfb-719530d4f47b | instance-000054cf | running |
 | 4  | af24b084-59c1-423c-8268-e84406ed8686 | instance-00002705 | running |
 | 5  | 7fa916f1-3b1c-4d5f-9d0b-9690717aca00 | instance-00002703 | running |
 | 6  | 539e53f9-5d17-4d2d-a2f7-b193e83e5fb0 | instance-00001c45 | running |
 | 7  | e57c0a32-8463-f8a8-b33e-9dbf61c1127e | instance-00000977 | running |
 +----+--------------------------------------+-------------------+---------+

 real    0m1.450s
 user    0m0.207s
 sys     0m0.031s

and without this patch:

 $ time hivemind -H np-rcc55 libvirt.list
 [np-rcc55] Executing task 'list'
 [np-rcc55] run: virsh list
 [np-rcc55] virsh list success

 +----+--------------------------------------+-------------------+---------+
 | ID |                 UUID                 |        Name       |  State  |
 +----+--------------------------------------+-------------------+---------+
 | 2  | a6b092eb-58bf-41db-b751-6919ca27ed86 | instance-00008ddf | running |
 | 3  | dda6d997-7e35-4e70-8cfb-719530d4f47b | instance-000054cf | running |
 | 4  | af24b084-59c1-423c-8268-e84406ed8686 | instance-00002705 | running |
 | 5  | 7fa916f1-3b1c-4d5f-9d0b-9690717aca00 | instance-00002703 | running |
 | 6  | 539e53f9-5d17-4d2d-a2f7-b193e83e5fb0 | instance-00001c45 | running |
 | 7  | e57c0a32-8463-f8a8-b33e-9dbf61c1127e | instance-00000977 | running |
 +----+--------------------------------------+-------------------+---------+

 real    0m42.495s
 user    0m41.336s
 sys     0m0.046s

Change-Id: I10e59eec1dfc478802692947f9551ffba44d6228
Merge "speed up operations by disabling known_hosts"