Allow ENIs to be allocated at a faster rate than 1 per 15 seconds

[nckturner]

Currently, the fastest IPAMD is willing to allocate ENIs is 1/15 seconds (5 seconds per loop + 10 seconds per ENI sleep time), but this can increase up to a max of 55 seconds if the ENI takes longer to attach. This could be improved.

[jaypipes]

As mentioned on #405, the 2.0 CNI plugin design will alleviate this issue. See aws/containers-roadmap#398 for more details. Closing this issue for the 1.x CNI series.