Optimize check for RHEL GPG key

Reduces the time to check for presence of
gpg pubkey for RabbitMQ from 1.6s to 0.1s.

Comparison:
$ time rpm -q gpg-pubkey-056e8e56-468e43f2a
package gpg-pubkey-056e8e56-468e43f2a is not installed

real	0m0.116s
user	0m0.008s
sys	0m0.004s

$ time test `rpm -qa | grep gpg-pubkey-056e8e56-468e43f2 | wc -l` -eq 0

real	0m1.650s
user	0m1.569s
sys	0m0.078s