Make changes on backup command builder

[ShahriyarR]

There are too many duplicated code in xtrabackup command builder functionality also I am using hasattr():

# Checking if extra options were passed:
            if hasattr(self, 'xtra_options'):
                args += " "
                args += self.xtra_options

            # Checking if extra backup options were passed:
            if hasattr(self, 'xtra_backup'):
                args += " "
                args += self.xtra_backup

This can be achieved by using try/except.
Also these sections can be grouped together maybe. Will see.

[ShahriyarR]

Phase 1 is over and merged.
Will be released as part of v1.5.4