build all does not ignore disabled providers

[mtaylor]

set up a tim / conductor system with three providers, when I disabled two if them and clicked on the image build all button, all three providers had image builds attempted which should not be the case.

Copying message from Tim Issue: aeolus-incubator/tim#101

[n1zyy]

The build step is at the ProviderType level, not the Provider level. You shouldn't be able to push to a disabled Provider, but Build All doesn't do that.

Should we decline to build target images for a ProviderType where all associated Providers are disabled? If so, should we not show that type in the build UI?