Default node selection override for given proposal not allowed #2018

Description:
Default node selection override for given proposal not allowed

Version/Build: mesa-1.6_openstack-os-build.3508.dev

Browsers Used: IE11.0

Reproduction Steps:

1. Using the crowbar GUI, try to create a proposal for either Glance, keystone, nova, etc
2. The proposal would list the default nodes recommended for the given proposal.
3. Try to remove the default node mentioned and try adding some other node from the node list.
4. The moment a new non-default node is added to the proposal, there is a message listed stating "Warning: Deleted" in the nodes area and a message "You must remove this node in order to save the proposal" at the top of the proposal
5. If we try to save the proposal, the recently added non-default node gets removed from the proposal.
6. Hence there is no way to add a non-default node for most of the barclamp proposals.

Observed Results:
Default node selection override for given proposal not allowed

Expected Results:
There should be a way to edit the proposal to add non-default nodes for any given proposal

Impact:
Proposals cannot be created with non-default nodes