add enable_app_replacement param in app_manager

[knorth55]

I add enable_app_replacement parameter in app_manager.
This parameter enable us to control whether app_manager denies overwriting app or not.
In the following example, I set enable_app_replacement false, and app_manager denied go_to_kitchen app because time_signal app is still running.

fetch@fetch15:~/ros/indigo/src/jsk-ros-pkg/jsk_robot$ rosservice call /fetch15/start_app "name: 'jsk_fetch_startup/time_signal'" 
started: True
error_code: 0
message: app [jsk_fetch_startup/time_signal] started
namespace: /fetch15/application
fetch@fetch15:~/ros/indigo/src/jsk-ros-pkg/jsk_robot$ rosservice call /fetch15/start_app "name: 'jsk_fetch_startup/go_to_kitchen'" 
started: False
error_code: 511
message: app [jsk_fetch_startup/go_to_kitchen] is denied because app [jsk_fetch_startup/time_signal] is already running.
namespace: /fetch15/application
fetch@fetch15:~/ros/indigo/src/jsk-ros-pkg/jsk_robot$ 

cc. @k-okada

[knorth55]

I updated this PR to follow your advice.

[v4hn]

Thanks @knorth55 .

@knorth55 heads-up from my side: my family just grew and now includes a new crying little kid.
So I will not have time to look at PR2 stuff over the next few months.
@k-okada please try to review new/open reviews from your lab and feel free to merge them if you consider them adequate.

[knorth55]

@v4hn thank you for merging this PR.
OK, I will ask @k-okada for other PRs.