Added a source column to the user table.
Source now has only two values UNKNOWN,LDAP with UNKNOWN being the
default and is an enum is com.cloud.User.
When the source is UNKNOWN, the old method of authenticating against all
the available authenticators is used. If a source is available, only
that particular authenticator will be used.
added overloaded methods in AccountService to createUserAccount and
createUser with source specified.
(cherry picked from commit 5da733072e838c830720909ef5ba27ef6ce6d0b0)
