Add mock to requirements.txt
Add mock to requirements.txt

Package mock is absent for functest, while it is required by
decorators.py.

Change-Id: I0d042def0335e369c4e22a349e3068829e1d2c59
Signed-off-by: Linda Wang <81613bebe84fa394bbc7c5cc1c21989c9bff2c52@huawei.com>

mywulin/functest,opnfv/functest,mywulin/functest,opnfv/functest