puppet: Switch double quoted strings to single quoted.
puppet: Switch double quoted strings to single quoted.

Resolves these warnings from puppet-lint.

puppet-lint| puppet/zulip/manifests/app_frontend_base.pp - WARNING: double quoted string containing no variables on line 14
puppet-lint| puppet/zulip/manifests/app_frontend_base.pp - WARNING: double quoted string containing no variables on line 19

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulipchat.com>

shubhamdhama/zulip,timabbott/zulip,brainwane/zulip,shubhamdhama/zulip,kou/zulip,brainwane/zulip,timabbott/zulip,eeshangarg/zulip,punchagan/zulip,synicalsyntax/zulip,showell/zulip,rht/zulip,kou/zulip,zulip/zulip,rht/zulip,shubhamdhama/zulip,timabbott/zulip,timabbott/zulip,shubhamdhama/zulip,andersk/zulip,rht/zulip,andersk/zulip,kou/zulip,hackerkid/zulip,andersk/zulip,synicalsyntax/zulip,eeshangarg/zulip,showell/zulip,eeshangarg/zulip,timabbott/zulip,punchagan/zulip,rht/zulip,brainwane/zulip,hackerkid/zulip,hackerkid/zulip,kou/zulip,showell/zulip,andersk/zulip,zulip/zulip,zulip/zulip,andersk/zulip,zulip/zulip,brainwane/zulip,punchagan/zulip,hackerkid/zulip,hackerkid/zulip,shubhamdhama/zulip,showell/zulip,kou/zulip,synicalsyntax/zulip,punchagan/zulip,synicalsyntax/zulip,zulip/zulip,timabbott/zulip,zulip/zulip,synicalsyntax/zulip,synicalsyntax/zulip,hackerkid/zulip,eeshangarg/zulip,zulip/zulip,brainwane/zulip,kou/zulip,showell/zulip,shubhamdhama/zulip,rht/zulip,punchagan/zulip,brainwane/zulip,rht/zulip,showell/zulip,eeshangarg/zulip,brainwane/zulip,eeshangarg/zulip,synicalsyntax/zulip,eeshangarg/zulip,showell/zulip,kou/zulip,andersk/zulip,punchagan/zulip,andersk/zulip,shubhamdhama/zulip,hackerkid/zulip,rht/zulip,punchagan/zulip,timabbott/zulip