tools: significantly simplify frr-reload context processing

tools: significantly simplify frr-reload context processing

Currently, in frr-reload we:
- store a list of single-line context keywords which needs to be
  frequently updated,
- have a separate "if" clause for every node and subnode we have in FRR.

Instead, we can store the tree of all known FRR nodes. This tree needs
to be updated whenever we add a new node, which is not frequent. And,
most importantly, it allows us to write node-agnostic code and save more
than 250 LOC.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>