fix "no tty" issue with sudo by using su

the use of `sudo` creates an issue with default sudo installations on CentOS 6: sudo is expecting a tty, which puppet might not be able to provide. `sudo` might also not be installed by default on minimal-install systems - this patch takes care of both discussed issues.

for reference:
```bash

Debug: Exec[checkmk_refresh_infra](provider=posix): Executing 'sudo -i -u infra /opt/omd/sites/infra/bin/cmk -O'
Debug: Executing 'sudo -i -u infra /opt/omd/sites/infra/bin/cmk -O'
Notice: /Stage[main]/Monitoring/Omd::Site[infra]/Exec[checkmk_refresh_infra]/returns: sudo: sorry, you must have a tty to run sudo
Error: /Stage[main]/Monitoring/Omd::Site[infra]/Exec[checkmk_refresh_infra]: Failed to call refresh: sudo -i -u infra /opt/omd/sites/infra/bin/cmk -O returned 1 instead of one of [0]
Error: /Stage[main]/Monitoring/Omd::Site[infra]/Exec[checkmk_refresh_infra]: sudo -i -u infra /opt/omd/sites/infra/bin/cmk -O returned 1 instead of one of [0]
```