On edit of replica pool manifest, operator should resize pool accordingly

[ftab]

From discussion on slack

Steps:

1. kubectl edit -n rook pool replicapool
2. Change spec\replication\size parameter to something else, say 3

Expected result:
It does the ceph osd pool set replicapool size 3 for you

[travisn]

note that the size can only be changed for pools that are already configured for replication. If you have an erasure-coded pool, you won't be able to change it to a replicated pool.

[kokhang]

In K8S 1.8, CRDs have validation we can leverage for this. kubernetes/kubernetes#47263

[galexrt]

@kokhang Are you sure we already want to use this feature? The docs read: This feature is alpha in v1.8 and may change in backward incompatible ways.. See the PR
for the validation docs here: kubernetes/website#5290