implemented correct validation for bucket size. Ref. bug 761
implemented correct validation for bucket size. Ref. bug 761

As discussed in wave discussion for March 10 we validate new bucket
size against unreserved physical memory of smallest node in cluster.

I've also added small fixed margin of 128MB of memory (for OS and our
non-bucket needs).

Signed-off-by: Matt Ingenthron <0b8dd71424cffcab543578047c2fbe6fd032151b@cep.net>

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