Provide the option to rollback to previous main.tf configuration when a build error happens

[cmd-ntrf]

A user tried to change the instance type from c2-7.5gb-31-avx2 to g1-8gb-c4-22gb. This triggered on Terraform side the following error:

Error: Error waiting for instance (80f2fbd3-57ea-4426-a51b-0b7dfaf1bb71) to resize: unexpected state 'ACTIVE', wanted target 'VERIFY_RESIZE'. last error: %!s(<nil>)

which indicates that OpenStack is unable to migrate the c2 instance to a g1 instance.

While I don't think it would have been possible to completely prevent the user from trying to migrate. MC-Hub could have done better on two aspects:

1. It should have reported the entire error message so the user would have had a better idea of what was happening.
2. MC-Hub should have provided the ability to the user to rollback its configuration to the most recent one that was built correctly.

As it is currently implemented, after the user press the apply button, the configuration that triggered a build error is saved in the database and the preceding one cannot be recovered. This leaves the user will only one choice to remember what the previous configuration was and try to fix it manually by trial and error.