Access the 'create' via importing from mbed_ls_tools instead of detect
Access the 'create' via importing from mbed_ls_tools instead of detect

Previously htrun target_base python module was importing the detect and
called `detect.create()` but recent standalone greentea htrun ( PR#302)
changes replaced importing of detect with mbed-lstools.main which
allows to create call directly instead of detect.create so updated the
target_base python to call create directly.

ARMmbed/greentea