slaprunner: review $PATH and integrate mosh

Following up !164 (merged) , this enables mosh in slaprunner.

At the same time this merge request contains changes so that:

• $PATH is defined consistently for all services, in a single place in the profile
• $PATH is not influenced by the current environment of the user running slapos or the system default. /usr/bin:/bin/ is still present in $PATH, but at the end, so that we fallback to system for commands not provided by slaprunner.

This also changes the way slapos command select the wrapper controlling web runner introduced !158 (merged) by adding ~/bin/ to the $PATH in ~/.bash_profile and no longer in a new ~/.bashrc

After these changes, we can use mosh with:

LC_ALL=C.UTF-8 mosh slapuser0@2001:67c:?:?:?:? --ssh='ssh -p 22222'

or if we set a ~/.ssh/config with:

host slaprunner_mosh
 user slapuser0
 port 22222
 hostname 2001:67c:?:?:?:?

simply:

LC_ALL=C.UTF-8 mosh slaprunner_mosh