mgr/dashboard: do not use "l" for variable name

mgr/dashboard: do not use "l" for variable name

see also https://www.flake8rules.com/rules/E741.html

also silences flake8 warnings like:

2: {tty:'./controllers/home.py:90:26: E741 ambiguous variable name 'l'':'./controllers/home.py:90:26: E741 ambiguous variable name 'l''}
2: {tty:'./controllers/logs.py:52:13: E741 ambiguous variable name 'l'':'./controllers/logs.py:52:13: E741 ambiguous variable name 'l''}
2: {tty:'./services/ganesha.py:36:40: E741 ambiguous variable name 'l'':'./services/ganesha.py:36:40: E741 ambiguous variable name 'l''}
2: 3     E741 ambiguous variable name 'l'

Signed-off-by: Kefu Chai <kchai@redhat.com>