Fallback to dynamic loader even if HADOOP_HDFS_HOME is not defined (#19336)

* Fallback to dynamic loader even if HADOOP_HDFS_HOME is not defined

Prior to this commit HadoopFileSystem required HADOOP_HDFS_HOME to be
defined to initialize the filesystem, even if libhdfs.so is located
outside of the standard location. This limitation is unnecessary and
can be safely removed.

As a nice side-effect, the error message is now more informative.

Before:

    Environment variable HADOOP_HDFS_HOME not set

After:

    libhdfs.so: cannot open shared object file: No such file or directory

Change-Id: Ief6a8679d7ef353003aa387f7767ebaa8ef290ce

* Addressed review comments

Change-Id: I703d57e022744e26d1b47732beeaa48c073bd5fc