Using _append line with rc.eos breaks rc.eos

[phil-dileo]

If you are the first to write to rc.eos then you want #!/bin/bash as the first line, but with the current code

    def append_rc_eos_lines(self, lines):
        '''Add lines to rc.eos.
        Args:
            lines (list): List of bash commands
        '''
        if not self._has_rc_eos():
            lines = ['#!/bin/bash'] + lines
        self._append_lines(RC_EOS, lines)

    def _append_lines(self, filename, lines):
        with open(filename, 'a') as output:
            output.write('\n')
            output.write('\n'.join(lines))

There will always be a newline as the first line in rc.eos. This breaks the shell script.