pyglib: flags: add function for getting value of a flag
pyglib: flags: add function for getting value of a flag

Reduce code duplication by creating a GetValue local function which
returns value for given flag.

	Change on 2016/02/15 by mpn <mpn@google.com>

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=114703335

google/python-gflags