InteractVty: re.escape() the VTY prompt in connect()

The VTY prompt may contain regular expression metacharacters in it,
so the self.re_prompt will not match as expected.  One example is
the modem app in osmocom-bb.git using promot 'OsmocomBB(modem)'.

Change-Id: I47b1f1c1765acb1b935dcbcc54bbb5eb539db4f0
1 file changed