gfapi: glfs_set_volfile_server() now entertains multiple calls

Previous API:

glfs_set_volfile_server (..., const char *host, ...) - single call

New API's:

glfs_set_volfile_server (..., const char *host1, ...)
glfs_set_volfile_server (..., const char *host2, ...)

Multiple calls to this function with different volfile servers,
port or transport-type would create a list of volfile servers
which would be polled during `volfile_fetch_attempts()`

glfs_unset_volfile_server (..., const char *host, ...) to remove
a server from the list (this is provided for future usage)

Change-Id: I313efbd3efbd0214e2a71465f33195788df406cc
BUG: 986429
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/7317
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>