Set gearman timeout to 300

In practice we are seeing that geard can occasionally get disrupted
and then temporarily backlogged enough that it exceeds the 30 second
timeout for submitting a job.  To make Zuul less fragile in this case,
increase the timeouts for any requests submitted to gearman.

Change-Id: I12741bb259c1a78fa2446d764318f84df34bac67