forbid clustering of 1.8.1 and 1.7.1.1. MB-5194
forbid clustering of 1.8.1 and 1.7.1.1. MB-5194

So that users who don't read release notes can not shoot themselves in
foot.

Change-Id: I74d44a1c8f875d534d8e62abc6f4f3025a5c0757
Reviewed-on: http://review.couchbase.org/15481
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

nimishzynga/ns_server,daverigby/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,pavel-blagodov/ns_server,nimishzynga/ns_server,vzasade/ns_server,ceejatec/ns_server,ceejatec/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,pavel-blagodov/ns_server,ceejatec/ns_server,daverigby/ns_server,daverigby/ns_server,vzasade/ns_server,mschoch/ns_server,membase/ns_server,daverigby/ns_server,vzasade/ns_server,vzasade/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,pavel-blagodov/ns_server,membase/ns_server,t3rm1n4l/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,mschoch/ns_server,membase/ns_server,daverigby/ns_server,mschoch/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,mschoch/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server