[FAB-3107] Replace 'user' with 'identity'

Replace all references to 'user' with 'identity' in the doc
Also, updated fabric-ca-server and fabric-ca-client command
help, removed yml comments that said certfiles property is
comma separated values.

Change-Id: I2d399634080a1533b77af596b4296c6dbd856911
Signed-off-by: Anil Ambati <[email protected]>

Author: Anil Ambati