Add job expiration dates

Add job expiration dates

This feature has two parts:
* Specifying expiration dates when scheduling test runs
* A global maximum age

Expiration dates are provided by passing `--expire` to `teuthology-suite` with
a relative value like `1d` (one day), `1w` (one week), or an absolute value like
`1999-12-31_23:59:59`.

A new configuration item, `max_job_age`, is specified in seconds. This defaults
to two weeks.

When the dispatcher checks the queue for the next job to run, it will first
compare the job's `timestamp` value - which reflects the time the job was
scheduled. If more than `max_job_age` seconds have passed, the job is skipped
and marked dead. It next checks for an `expire` value; if that value is in the
past, the job is skipped and marked dead. Otherwise, it will be run as usual.

Signed-off-by: Zack Cerza <zack@redhat.com>