bf install

[beczkowb]

Requirements management became a bit confusing. Maybe we should merge the following commands:

bigflow codegen pin-dataflow-requirements
bf build-requirements
pip install -r resources/requirements.txt

Into the single command:
bf install

[anjensan]

Also new flag --sync (or --install) might be added to build-requirements.
Command codegen pin-dataflow-requirements also might be merged as a flag:

bigflow build-requirements --pin-dataflow --sync