allow 'main' as valid name for find_test_caller

find_test_caller assumed that the rest clients were only ever run
under testr. With the introduction of javelin into the tempest
tree this will not be true. 'main' is a completely valid name
for well formed programs, so make that an option as well.

Prereq for blueprint:javelin2

Change-Id: I9a591eaa1cf4dabba58f06a64814611a05a51365
Merge "allow 'main' as valid name for find_test_caller"