add a % sign to percentage in server log warning

[mtbc]

What this PR does

Fixes the "servants used for session" log messages reported by @kennethgillen.

Testing this PR

Search the Blitz log's warnings. If it is too hard to trigger the warning then it is probably easiest to comment out the preceding "if" condition then do a local build.

Related reading

https://trello.com/c/KSZ8Q2Rc/53-logging-of-servants

[kennethgillen]

Thank you @mtbc!

[pwalczysko]

After commenting the lines 178 and 181 of https://github.com/openmicroscopy/openmicroscopy/pull/5536/files#diff-b609591712a94918861a8cea48d6a26aL178 out, stopping server and running ./build.py build-server, then starting server again I get

cat var/log/Blitz-0.log | grep servants
2017-10-10 11:31:34,066 WARN  [                omero.util.ServantHolder] (l.Server-4) 0% of servants used for session ba51bef4-6e9b-4026-9f03-133e1a112b27
2017-10-10 11:31:35,206 WARN  [                omero.util.ServantHolder] (l.Server-8) 0% of servants used for session c2bf2000-c15c-4b00-9508-866728b947a1
2017-10-10 11:31:35,215 WARN  [                omero.util.ServantHolder] (l.Server-3) 0% of servants used for session ca115514-8158-46fe-95d0-cde53adc964c
2017-10-10 11:31:35,274 WARN  [                omero.util.ServantHolder] (l.Server-7) 0% of servants used for session c2bf2000-c15c-4b00-9508-866728b947a1
...

this seems okay. I wonder that all the percents numbers are zero, but this is possibly because I commented out the if ?

[mtbc]

Yes, I think just none of them get very far in the range; we usually hardly need any servants it seems.

[pwalczysko]

Ready to merge FMPOV