Mesos integration does not propagate Alluxio site properties

Expected:
When you run the mesos integration, your local alluxio-site.properties should be applied to the launched servers. We do this by passing an environment variable ALLUXIO_MESOS_SITE_PROPERTIES_CONTENT to Mesos. The variable should contain the content that we want to add to alluxio-site.properties.
Actual:
Instead of passing the content, we have been setting ALLUXIO_MESOS_SITE_PROPERTIES_CONTENT to a file path for alluxio-site.properties. The bug was introduced in 1.7

The fix should be pretty straightforward, just iterating over property keys to set the content correctly.