prow: approve: our robot can approve PRs?

[stevekuznetsov]

We recently moved to the assign/approve/blunderbuss trio of Prow plugins in our cluster and we're seeing our robot considered a valid approver for a PR like so: openshift/origin#17350 (comment)

[APPROVALNOTIFIER] This PR is **APPROVED**

This pull-request has been approved by: *<a href="https://github.com/openshift/origin/pull/17350#issuecomment-347988758" title="LGTM">openshift-merge-robot</a>*, *<a href="https://github.com/openshift/origin/pull/17350#" title="Author self-approved">rootfs</a>*, *<a href="https://github.com/openshift/origin/pull/17350#issuecomment-348004549" title="LGTM">stevekuznetsov</a>*

The full list of commands accepted by this bot can be found [here](https://github.com/kubernetes/test-infra/blob/master/commands.md).

<details >
Needs approval from an approver in each of these OWNERS Files:

- ~~[images/base/OWNERS](https://github.com/openshift/origin/blob/master/images/base/OWNERS)~~ [stevekuznetsov]

You can indicate your approval by writing `/approve` in a comment
You can cancel your approval by writing `/approve cancel` in a comment
</details>
<!-- META={"approvers":[]} -->

/area prow
/kind bug
/cc @Kargakis @cjwagner

[cjwagner]

I believe this is an interaction with the lgtm-after-commit-munger which comments with /lgtm cancel //PR changed after LGTM, removing LGTM...
The approve plugin considers this an approval because where it expects the argument cancel it finds cancel //PR changed after LGTM, removing LGTM....

I think the best fix is to change the approve plugin to check that the argument to the /lgtm command contains cancel instead of checking for strict equality. I'll make a PR.