Removing deprecated using of flags module from project

Moving file flags.py to manila/common/config.py,
replacing FLAGS by CONF. Rename modules fake_flags to conf_fixture,
test_flags to test_conf, declare_flags to declare_conf,
runtime_flags to runtime_conf like it was done in cinder, nova, glance etc.

Implement bp: use-oslo-conf

Change-Id: I38d869123e5e706d3b06f1844b97ead05e22668f