Prior to this the only way to configure the URL that LocalFileServer listens
to is via MESOS_NIMBUS_HOST environment variable.  That differs from the
behavior of the core MesosNimbus which supports that env variable, as well
as the nimbus.host field in the storm.yaml config.  So update the code to
make them equally configurable.

This is needed to allow the travis-ci builds to succeed.
It was failing with errors like this:

```
$ travis_retry sudo apt-get -o Dpkg::Options::="--force-confnew" install -y docker-engine=${TRAVIS_DOCKER_VERSION}
Reading package lists... Done
Building dependency tree
Reading state information... Done
Recommended packages:
  yubico-piv-tool
The following packages will be DOWNGRADED:
  docker-engine
0 upgraded, 0 newly installed, 1 downgraded, 0 to remove and 15 not upgraded.
Need to get 8,725 kB of archives.
After this operation, 58.0 MB disk space will be freed.
E: There are problems and -y was used without --force-yes

The command "sudo apt-get -o Dpkg::Options::=--force-confnew install -y docker-engine=1.10.3-0~trusty" failed. Retrying, 2 of 3.
```