GlusterManager:
- add various error policies to gluster_call
- add set_vol_option method, with optional error tolerance
(making use of the respective gluster call error policy),
with support for Boolean options
- rename get_gluster_vol_option method to get_vol_option
for uniform nomenclature and simplicity (the "gluster" in
the method name was redundant as the classname already
hints about the Gluster scope)
Partially implements bp gluster-code-cleanup
Change-Id: I02a1d591d36c6a64eea55ed64cf715f94c1fd1c8
5847a12260