add flag to set the host interface to use

[mmiller1]

This addresses issue #13 by allowing a user to specify an optional --iface flag that overrides the default NIC on the host.

[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

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[coveralls]

Coverage decreased (-0.1%) to 17.031% when pulling 1a032a0 on msm93v2:master into fd228e4 on aledbf:master.

[aledbf]

@msm93v2 please sign the CLA

[mmiller1]

As far as I can tell I have already done that, clicking the CLAassistant bots link greets me with this message
"You have signed the CLA for aledbf/kube-keepalived-vip"

[coveralls]

Coverage decreased (-0.1%) to 17.031% when pulling 1a032a0 on msm93v2:master into fd228e4 on aledbf:master.