LU-3425 utils: rename l_getgroups.8 to l_getidentity.8

LU-3425 utils: rename l_getgroups.8 to l_getidentity.8

Rename the man page for l_getgroups.8 to l_getidentity.8 to match
the "new" name of the utility, and update the content accordingly.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I698c8170cb0623ff692bfe76962b5c71d75bc18b
Reviewed-on: https://review.whamcloud.com/24219
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Steve Guminski <stephenx.guminski@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>