InfluxDB with Let's Encrypt Certs failed to find PEM block

### Directions

As asked by Sean Beckett a bug report (or feature request). See https://groups.google.com/forum/#!topic/influxdb/2k6fV_giunA

### Bug report

CentOS 7

1. Install server with hostname, install webpage and configure Let's Encrypt, eg https://m01.outsideit.net/
2. Install InfluxDB, fix permissions (yes after fresh installation I always need to fix some permissions on /var/lib/influxdb/meta)
3. Use a influxdb.conf with something like this:
```
[admin]
enabled = true
bind-address = ":8083"
https-enabled = true
https-certificate = "/etc/letsencrypt/live/m01.outsideit.net/fullchain.pem"
https-private-key = "/etc/letsencrypt/live/m01.outsideit.net/privkey.pem"

[http]
enabled = true
bind-address = ":8086"
auth-enabled = true
log-enabled = true
write-tracing = false
pprof-enabled = false
https-enabled = true
https-certificate = "/etc/letsencrypt/live/m01.outsideit.net/fullchain.pem"
https-private-key = "/etc/letsencrypt/live/m01.outsideit.net/privkey.pem"
```
__Expected behavior:

`sudo systemctl start influxdb`

results in started service

__Actual behavior:

influxdb service doesn't start

__Additional info:

When trying manually

`/usr/bin/influxd -config /etc/influxdb/influxdb.conf`

I can see the following error:

`run: open server: open service: tls: failed to find PEM block with type ending in "PRIVATE KEY" in key input after skipping PEM blocks of the following types: [CERTIFICATE CERTIFICATE]`

But

```
cat /etc/letsencrypt/live/m01.outsideit.net/privkey.pem
-----BEGIN PRIVATE KEY-----
mycertdarfkzofkjzeifjezfjiozejfaefakjdop
```

I can make it work when I just concatenate the fullchain and the privkey pems but as Let's Encrypt certificates have to be renewed every three months it would be nice if this just worked automatically with the certificates from the live folder.

Start by reproducing the failure with /usr/bin/influxd -config /etc/influxdb/influxdb.conf and the separate fullchain.pem and privkey.pem paths shown. Trace the TLS certificate and private-key loading, then verify that the service starts without concatenating the PEM files and continues to work after renewal in the live folder.

Written by the indexing model from the issue text.