Proposal: change water_level_high environment variable to water_level

[sp4ghet]

This would allow us to process water_level as a single environment variable rather than a bundle of different variables heavily related to the sensor configuration. Currently there's only 4 logical states, with 3 possible states:

lower: DRY upper: DRY
lower: WET upper: DRY
lower: WET upper: WET
lower: DRY upper: WET //impossible (unless something is wrong)

If we can encode these states into some Float value or an enum, and run some kind of feedback loop on it, it would allow for us to do some drought stressing at the later phases of the grow cycle or some other interesting actuation.

[rbaynes]

I like this proposal.