Fix missing min_avail_mem in memory governor

Somehow the memory governor made it into without the needed
min_avail_mem variable. Add that to prevent repeated crashing of the
governor thread [1].

[1] Trace:
2018-02-01 14:42:15,533 ERROR zuul.ExecutorServer: Exception in governor thread:
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/zuul/executor/server.py", line 1959, in run_governor
    self.manageLoad()
  File "/usr/lib/python3.6/site-packages/zuul/executor/server.py", line 1975, in manageLoad
    elif avail_mem_pct < self.min_avail_mem:
AttributeError: 'ExecutorServer' object has no attribute 'min_avail_mem'

Change-Id: I9af5ba3ef3200f1a8d1bdb41906a1b77ef4e7f55
1 file changed