logging: Expose set_log_file_path() in libglusterfs.

This patch also changes the behavior of glfs_set_logging().
If logfile argument is not provided to glfs_set_logging(),
libgfapi uses set_log_file_path() to create a logfile.

Change-Id: I49ec66c7f16f5604ff2f7cf7b365b08a05b5460d
BUG: 764890
Signed-off-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/5910
Reviewed-by: Anand Avati <avati@redhat.com>
Tested-by: Anand Avati <avati@redhat.com>