let router bootstrapper get token from a path

[qrkourier]

The Linux and Docker router deployments employ an environment variable ZITI_ENROLL_TOKEN that must be the erott.

Some users have tried to provide the token by defining this variable with the filesystem path to the token instead of the token string itself.

Let the router's bootstrap.bash handle this scenario gracefully by checking if the value is a non-empty file. If so, slurp the file. If not, assume it is the erott as a string.