mon/MOnCap: fix command parsing

Signed-off-by: Sage Weil <sage@inktank.com>
mon/MonCap: add 'allow profile <foo> x'

Signed-off-by: Sage Weil <sage@inktank.com>
mon/MonCap: revise grammar

 allow rwx
 allow service rwx
 allow profile foo
 allow command bar
 allow command bar with k1=v1 k2=v2

Signed-off-by: Sage Weil <sage@inktank.com>