New regression: set-env no longer accepts empty values

[Aaron Bentley]

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I have discovered a regression in juju set-env.  I have filed a bug
accordingly:

https://bugs.launchpad.net/juju-core/+bug/1388073


A recent change to juju broke set-env, so that it no longer accepts
empty values.

In old versions this worked:
juju --show-log set-env -e kvm-upgrade-trusty-amd64 tools-metadata-url=

In new versions, it gives:
error: expected "key=value", got "tools-metadata-url="

Revision f81cc435 is the first version that our tests show exhibits
this bug. We also know that 289da217 did not exhibit this bug.

This is a regression because
- - It breaks commandline compatibility with other juju releases
- - It breaks CI
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