master: Isolate personality management in HA cluster

Metadata servers in a given LizardFS installation can be managed in two
ways. One possibility is to manage them by hand and in that case an
administrator should use mfsmaster.cfg entry

    PESONALITY = (master|shadow)

The other possibility is to manage servers automatically with use of HA
cluster mechanism. If user wants to do that she has to use (newly added
in this commit) config option

    PESONALITY = ha-cluster-managed

together with (also newly added) command line option

    mfsmaster -o ha-cluster-personality=(master|shadow) ...

If HA cluster decides to promote metadataserver to 'master' state it
should use just added lizardfs-admin command:

    lizardfs-admin promote-shadow ...

Change-Id: I2e781f4ef69a9cfd49736e21f8f66c34b5f211ff