ceph-nano instructions need to be updated for Nautilus release

[gsternagl]

ceph dashboard set-login-credentials "username" "password" => ceph dashboard set-login-credentials -i "password-file"

ceph dashboard set-rgw-api-access-key "$RGW_ACCESS" => ceph dashboard set-rgw-api-access-key -i "file with RGW_ACCESS key"

ceph dashboard set-rgw-api-secret-key "$RGW_SECRET" => ceph dashboard set-rgw-api-secret-key -i "file with RGW_SECRET"