rhel, xenserver: Run unit tests while creating rpms.

For RHEL, Fedora and Xenserver, run unit tests while
building rpms.  This may catch some cross-platform bugs.

The commit also allows the users to optionally skip unit tests.
(On debian, the default is to run unit tests. For consistency,
do the same for rpms.)

VMware-BZ: 1267127

CC: Flavio Leitner <fbl@redhat.com>
CC: Ben Pfaff <blp@nicira.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Flavio Leitner <fbl@redhat.com>
Tested-by: Flavio Leitner <fbl@redhat.com>