testnode: drop gperftools-devel

testnode: drop gperftools-devel

Stop installing the gperftools-devel package on testnodes, for the
following reasons:

1. Ceph does not require gperftools-devel at runtime, so installing it
   unnecessarily just slows everything down at this point.

2. When we want to test newer builds of gperftools (say, from RHEL 7.3),
   if the corresponding "gperftools-devel" package is not available on
   the test node, then yum will fail the transaction.

(Additionally, gperftools-devel is no longer in EPEL 7, and it has moved to
RHEL 7 Optional in RHEL 7.2 (https://bugzilla.redhat.com/1213879,
https://access.redhat.com/errata/RHEA-2015:2293). The "epel" list is no
longer the correct list for this package on redhat_7 and centos_7.)