Fix nginx agent

When use the nginx agent with OCF_CHECK_LEVEL=20, we can specify the
parameter 'testconffile'. But now the agent file of nginx read
'test20conffile'. So change the name of parameter to 'test20conffile'.

Signed-off-by: KATOH Yasufumi <karma@jazz.email.ne.jp>
Merge pull request #451 from tenforward/fix-nginx-ra

Fix nginx agent