Description of problem: Heketi is not creating a volumes when we are giving very large average file size when we are creating arbiter volume. It is because of the xfs minimum brick size condition is not met.

Version-Release number of selected component (if applicable): 6.0.0.12

How reproducible:

Steps to Reproduce:
1. heketi-cli volume create --size=10 --gluster-volume-options='user.heketi.arbiter true,user.heketi.average-file-size 1024'

Actual results: 
Error: Unable to execute command on glusterfs-storage-dz65l: agsize (3072 blocks) too small, need at least 4096 blocks
Usage: mkfs.xfs
/* blocksize */		[-b log=n|size=num]
/* metadata */		[-m crc=0|1,finobt=0|1,uuid=xxx]
/* data subvol */	[-d agcount=n,agsize=n,file,name=xxx,size=num,
			    (sunit=value,swidth=value|su=num,sw=num|noalign),
			    sectlog=n|sectsize=num
/* force overwrite */	[-f]
/* inode size */	[-i log=n|perblock=n|size=num,maxpct=n,attr=0|1|2,
			    projid32bit=0|1]
/* no discard */	[-K]
/* log subvol */	[-l agnum=n,internal,size=num,logdev=xxx,version=n
			    sunit=value|su=num,sectlog=n|sectsize=num,
			    lazy-count=0|1]
/* label */		[-L label (maximum 12 characters)]
/* naming */		[-n log=n|size=num,version=2|ci,ftype=0|1]
/* no-op info only */	[-N]
/* prototype file */	[-p fname]
/* quiet */		[-q]
/* realtime subvol */	[-r extsize=num,size=num,rtdev=xxx]
/* sectorsize */	[-s log=n|size=num]
/* version */		[-V]
			devicename


Expected results: It should create arbiter brick with minimum size.