LU-5022 build: add rhel7 to server build selections

LU-5022 build: add rhel7 to server build selections

Add build files to allow building el7 server

Test-Parameters: clientdistro=el7 mdsdistro=el7 ossdistro=el7 \
 mdsfilesystemtype=ldiskfs mdtfilesystemtype=ldiskfs \
 ostfilesystemtype=ldiskfs testgroup=review-ldiskfs \
 envdefinitions=SANITY_EXCEPT=17m

Signed-off-by: Bob Glossman <bob.glossman@intel.com>
Change-Id: I3f6126473aaa95874bd2e225e79147ea23046f4e
Reviewed-on: http://review.whamcloud.com/12289
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Yang Sheng <yang.sheng@intel.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>