Also the following related changes:
* Removed Python 2 support because it's not supported by TF and related packages for a long time.
* Upgraded tensorboard and estimator packages to the required versions.
* Added extra plugins for tensorboard to support profiling.
* In the previous derivation versions, TF_SYSTEM_LIBS didn't have any effect because it was reset at repo fetching stage, so TF always used its own dependencies. Made TF_SYSTEM_LIBS actually work and fixed the errors caused by enabling it.
* Enabled tensorboard by default (but still keeping an option to disable it if needed).
