bundle: Support for handling PEM file containing the public key

The "verification-key" flag used by the `run` and `build` commands
should be able to handle a PEM file containing a public key.
Earlier we were not checking if the value of the flag represents
a file on disk. This change will check if the value points to a
file, then read it contents and set the public key accordingly.

Fixes: #2796

Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>