The rabbitmq module was using `__context__` to store paths of
rabbitmq executables. `__context__` may be cleared but the module
still could remain in use, in which case it would fail to work
correctly. Move the paths of the rabbitmq executables to their own
global variables so that they are not affected by the lifespan of
`__context__`.
Signed-off-by: Sergey Kizunov <sergey.kizunov@ni.com>
