Plugins are now disabled at startup time unless there is some relevant
configuration in hydra.conf. This avoids hydra-notify having to do a
lot of redundant work (a lot of plugins did a lot of database queries
*before* deciding they were disabled).
Note: BitBucketStatus users will need to add 'enable_bitbucket_status
= 1' to hydra.conf.
This adds a `InfluxDBNotification` plugin which is configured as:
```
<influxdb>
url = http://127.0.0.1:8086
db = hydra
</influxdb>
```
which will write a notification for every finished job to the
configured database in InfluxDB looking like:
```
hydra_build_status,cached=false,job=job,jobset=default,project=sample,repo=default,result=success,status=success,system=x86_64-linux build_id="1",build_status=0i,closure_size=584i,duration=0i,main_build_id="1",queued=0i,size=168i 1564156212
```
