Convert last_run to UTC epoch timestamp

[vinteo]

The devt returned from the API is in local time in the form of epoch timestamp. This should be converted to a proper UTC epoch timestamp using the timezone tz returned by the API. The time zone is in the format:

Time zone (floating point time zone value * 4 + 48). For example, GMT+0:00 is 48; GMT-4:00 is 32, GMT+9:30 is 86. Acceptable range is 0 to 108.

Context: vinteo/hass-opensprinkler#89