Add "verbose" option to "exec_command" function

[vmalyi]

Make "exec_command" function use verbose mode by default (function should output than stdout to console) and introduce a possibility to redirect stdout to DEVNULL in order to supress it.

Function signature should look like: exec_command(adb_full_cmd, verbose=True)

[vmalyi]

"check_output" doesn't support "stdout" argument, that's why there is no change to redirect it to os.devnull.

Another solution is to be found