remove whitespaces from (Ext)Desired_Sites classAd

As discussed in this elog
https://cms-logbook.cern.ch/elog/GlideInWMS/808

we should separate the site names in the jdl only by "," (commas with no whitespace), such that
```
DESIRED_Sites = "T2_CH_CERN, T1_US_FNAL"
```
will be
```
DESIRED_Sites = "T2_CH_CERN,T1_US_FNAL"
```
@ticoann please review (maybe @bbockelm as well)