Introduce more entropy in veth name generation

The current 4 chars are not enough, bumped to 7

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
Add loop for veth pair creation on ErrInterfaceExists

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
Merge pull request #212 from crosbymichael/veth-naming

Add more entropy to veth pair creation