[SPARK-17944][DEPLOY] sbin/start-* scripts use of `hostname -f` fail with Solaris

## What changes were proposed in this pull request?

Modify sbin/start-master.sh, sbin/start-mesos-dispatcher.sh and sbin/start-slaves.sh to use the output of 'uname' to select which OS-specific command-line is used to determine the host's fully qualified host name.

## How was this patch tested?

Tested by hand; starting on Solaris, Linux and macOS.

Author: Erik O'Shaughnessy <erik.oshaughnessy@gmail.com>

Closes #15557 from JnyJny/SPARK-17944.