[INF-1923] add check for already defined constant

[katjacresanti]

This change adds a check for whether or not ReplicaPools(class_name) is already defined before setting it again to mitigate warning messages in the sidekiq stderr logs:

warning: already initialized constant ReplicaPools::Admin1

Also bumps the minor release version to 2.6.7.

[talaris]

This should bump the version number as well to prep for release.