Applying proposal with non-default nodes fails with error '400 "Bad Request"'

[hgaikwad]

Description:
Applying proposal with non-default nodes fails with error '400 "Bad Request"'

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. If we try to apply the proposal, the proposal fails with the error '400 "Bad Request"'
5. Hence there is no way to add a non-default node for most of the barclamp proposals.

Observed Results:
Applying proposal with non-default nodes fails with error '400 "Bad Request"'

Expected Results:
There should be a way to edit the proposal to add non-default nodes for any given proposal, or atleast there should be a descriptive message explaining the exact problem

Impact:
Proposals cannot be created with non-default nodes