Add support for setting log level

[sammcj]

To change the glusterd global log level:

glusterd --log-level WARNING

This can also be set in the config file, so perhaps the module could just accept options in the form of an array similar to the volume level options that would then populate the config file?

 In the /etc/sysconfig/glusterd file, locate the LOG_LEVEL parameter and set its value to WARNING.

## Set custom log file and log level (below are defaults)
#LOG_FILE='/var/log/glusterfs/glusterd.log'
LOG_LEVEL='WARNING'

As per: https://access.redhat.com/documentation/en-US/Red_Hat_Storage/3.1/html/Administration_Guide/Configuring_the_Log_Level.html