Nailgun does not allow save settings without restricted unfilled plugin fields

[Igor Zinovik]

Environment:
Fuel 9.0 (build 249) with fuel nsxv plugin 3.0.
Any plugin with mandatory fields and regex restrictions will
also hit this case.

Problem description

Steps to reproduce:
 1. Create cluster with vCenter support
 2. Select Neutron with NSX
 3. Leave default Glance backend
 4. Switch into created, leave mandatory NSX plugin settings on Network
    tab empty
 5. Switch to Settings tab, select Storage section, switch Glance
    backend to VMware vSphere
 6. Click Save settings, following message will appear
    https://gyazo.com/fe611929d9e50887962ab649c848f7be

Expected result:
 Fuel allows settings non-plugin related settings without errors

Actual result:
 Error message that does mazes user.

Impact:
 Operator will not know that this message is plugin related, not settings
 related.

[root@nailgun ~]# shotgun2 short-report
cat /etc/fuel_build_id:
 249
cat /etc/fuel_build_number:
 249
cat /etc/fuel_release:
 9.0
cat /etc/fuel_openstack_version:
 mitaka-9.0
rpm -qa | egrep 'fuel|astute|network-checker|nailgun|packetary|shotgun':
 fuel-release-9.0.0-1.mos6341.noarch
 fuel-misc-9.0.0-1.mos8320.noarch
 fuel-mirror-9.0.0-1.mos133.noarch
 fuel-openstack-metadata-9.0.0-1.mos8668.noarch
 fuel-provisioning-scripts-9.0.0-1.mos8668.noarch
 python-fuelclient-9.0.0-1.mos312.noarch
 fuel-9.0.0-1.mos6341.noarch
 fuel-utils-9.0.0-1.mos8320.noarch
 fuel-nailgun-9.0.0-1.mos8668.noarch
 rubygem-astute-9.0.0-1.mos740.noarch
 fuel-library9.0-9.0.0-1.mos8320.noarch
 network-checker-9.0.0-1.mos72.x86_64
 fuel-agent-9.0.0-1.mos273.noarch
 fuel-ui-9.0.0-1.mos2676.noarch
 fuel-setup-9.0.0-1.mos6341.noarch
 nailgun-mcagents-9.0.0-1.mos740.noarch
 python-packetary-9.0.0-1.mos133.noarch
 fuelmenu-9.0.0-1.mos269.noarch
 fuel-bootstrap-cli-9.0.0-1.mos273.noarch
 fuel-migrate-9.0.0-1.mos8320.noarch
 shotgun-9.0.0-1.mos88.noarch
 fuel-notify-9.0.0-1.mos8320.noarch
 fuel-ostf-9.0.0-1.mos925.noarch