Integrate devfile/api validator in devfile/library

[yangcao77]

As redhat-developer/odo#4085 has been merged in api repo, the validator is ready to be integrated within devfile/library.

The validation functions should be called after the devfile has been parsed and flattened, and returns error if the devfile is invalid.

[yangcao77]

This issue still has project & starterproject validation left.

It is due to the bug in devfile/api: #339
Move this issue back to in progress since it is currently blocked by #339, will need another PR to adopt new change in devfile/api after 339 has been merged