Add --load-list argument to tempest to accept a non-regex whitelist

In tempest run
'-l/--list-tests' argument generates a non-regex list of all the tests
in a seperate line:
Added the argument '--load-list', to tempest run, that accepts a file
of non-regex whitelists like testr supports
Updated documentation for --load-list

Change-Id: I75488871ac797d6bec4e2834c9d1b0c7885c891b
Closes-Bug: #1700136
2 files changed