This function uses python's extend function for lists in order to
concatenate lists. So, given the input: [['v1', 'v2'], ['v3', 'v4']]
the result would be ['v1', 'v2', 'v3', 'v4'].
This comes as an alternative to using yaql's concat.
Change-Id: I082833e73388540b29f71b78293fd36fe04b7b92